Visser Three vs Slappy the Dummy

 

I have recently completed the two children's book series Animorphs and Goosebumps, the two series having a long connected history being both sold at Scholastic Book Fairs with some promotion between them, and targeting roughly the same audience. While Animorphs is mostly serialized and Goosebumps mostly episodic, both series feature a distinctive central antagonist who is known for being particularly evil. Visser Three is considered so evil that Elfangor is described as "sensing his evil" and said that Alloran's face "shone" with evil in The Andalite Chronicles, which, in a science-fiction series like Animorphs, are pretty dramatic statements of evil. Slappy in contrast is stated to be "evil itself", it is stated in The Bride of the Living Dummy that his soul is the pure evil spirit of the sorcerer who made him, and in his Splat Stats it said his evil is at a ranking of 9, the highest ranking of any animate thing given a ranking. Both also serve as an iconic character for their series, with Visser Three being the primary antagonist for nearly the entirety of Animorphs and Slappy having the most single appearances of any Goosebumps villain and being depicted as the leader of the Goosebumps monsters in all Goosebumps material. And both are known for enslaving others. So I think that leads to the obvious question...

Who'd win?

I am going to be separating this into three scenarios:

Scenario 1: A standard vs scenario, no prep or prior knowledge, standard forms, standard equipment for both  (Which for these two is functionally nothing) This will be the most involved explanation for both as I need to express both characters' capacities

Scenario 2: A prepped scenario where both show up prepared to fight to the death with all the equipment they could quickly get their hands on and with the knowledge that their opponent is a threat. 

Scenario 3: Both characters at their hypothetical strongest including questionably canon material and given full benefit of the doubt.

Victory is by death, knockout, or surrender.

An aside about low-ends:

Before I go on, I need to talk about the fact that both characters have a significant amount of low-end feats and embarrassing displays. Narratively these exist to allow the child-age to teenage protagonists to somehow defeat them and should most likely be considered Plot-Induced Stupidity.

Slappy and other Dummies in Goosebumps are pretty consistently physically equaled or defeated by completely normal humans. It's possible that these humans are just bizarrely powerful (in fairness Book 1 - Welcome to the Dead House has two normal kids push over a tree and Book 41 - Bad Hare Day has a child martial artist being able to kick in a locked door.) It's also possible that despite his showings, somehow Slappy is at the physical level of a normal child. However, I think the most reasonable interpretation is that these showings are just narrative conceits to allow the child protagonists to have agency in the narrative, and shouldn't delegitimatize Slappy's actual feats.

While Visser Three has no real physical low-end feats the same way, he does suffer from a dramatic number of mental low-end feats, regularly being outsmarted by a team of five teenagers. It's possible that the Animorphs are all superhuman mental prodigies (and in fairness even without Ax's involvement, Cassie and Tobias are both implied to be creations of the Ellimist for the purpose of defending Earth in his game with Crayak.) It's also possible that despite his status, Visser Three is just a complete idiot. However, I think the most reasonable interpretation is that these mental shortcomings are a combination of personality defects and necessities in the plot to allow the Animorphs to engage in guerilla warfare against an advanced alien race.

As such I'm not going to be holding either characters' various failures against children against them. This is a measurement of how strong and mentally the two "should" be, based on the things they've done in the narrative.

Origins of Evil:

Esplin 9466 is a Yeerk, a green parasitic organism from another world resembling slugs. The Yeerks are an alien race that infiltrate worlds and take any compatible bodies of the local denizens by crawling in their ear and physically controlling their brain. Once a Yeerk is in someone's brain, they can read all the memories and gain all the information of their host, mimicking them perfectly, and controlling them easily, with it taking extreme acts of willpower to overpower Yeerk control for even a moment. Their only organized opposition are the Andalites, an advanced alien race that consider themselves essentially the police of the galaxy and the Yeerks consider meddlers.

Esplin 9466 was a particularly ambitious Yeerk fascinated by the Andalites and succeeded at being the first, and for the most part only, Yeerk to get an Andalite Host Bost, that being of War Prince Alloran-Semitur-Corrass, an achievement that got him promoted to the third highest rank in the Yeerk military, Visser Three. Visser Three led the Yeerk infiltration of the Earth, even temporarily being promoted to Visser One, though was harassed the entire time by a guerilla campaign of the Animorphs and their allies.

Hundreds of years ago, the sorcerer Ephraim Darkwell lived in a small European village. With his magic he gave life to the lifeless, made dummies come alive. He promised he would use this only for the purposes of the good. But he was not believed. The local village came to burn him for his witchcraft, even attacking his beloved son Isaac, in reality another dummy brought to life by Ephraim's magic. In vengeance for the harm done to his "son" and the death they planned for him, he created another dummy and invested his evil into it. He named this creation "Slappy", a comedic name so no one would suspect him. And so whenever the sorcerer's magic words "Karru Marri Odonna Loma Molonu Karrano" are spoken, Slappy returns to life and continues to spread chaos and evil.

Despite his relatively small stature, in the secret world of horrific monsters Slappy's sheer evil, and the inherent magic his father left him with, allowed him to gain considerable power, even becoming a leader of Horrorland, a magical evil amusement park where various monsters collaborate. 

Physical Capabilities:

Visser Three has gained the powerful body of a military Andalite, giving him superhuman physical capabilities. While his body is no more durable or resistant to the environment than a human's for the most part, and his arms are actually a bit weaker than a normal human's, the Andalite body has several physical advantages. It has two stalk eyes that can rotate 360 degrees, completely invalidating some forms of stealth and in Megamorphs 2 - In the Time of Dinosaurs it's noted by Ax that his stalk eyes can see a bit further into the infrared and ultraviolent spectrums, seeing an invisible forcefield. In Book 52 - The Sacrifice Ax states that the speed of Andalites is like that of horses and in Megamorphs 1 - The Andalite's Gift, Marco estimates that Ax can run at 40 MPH (~17.88 meters per second.) 

However more notable than these is one of the three great weapons of the Andalites, the Andalite tail blade. The Tail Blade is an important part of Andalite culture to the point that an Andalite without one is considered an outcasted cripple in Book 40 - The Other and Andalite ships are designed with a tail to mimic the Tail Blade. The Tail Blade is a blade generally compared to that of a Scorpion and is stated in Book 9 - The Secret to be able to reach long enough to hit someone directly in front of Visser Three. The speed and precision of an Andalite's tail blade is extremely highly vaunted, able to hit any precise spot on an enemy's body and referred to consistently as faster than the eye. In Book 36 - The Mutation Jake is confident Ax could take down two human guards with his tail before they could even "think" about pulling the trigger on their gun. The Tail Blade also creates a sharp sound as it slices through the air and in Book 37 - The Weakness Ax's tail blade speed is compared to a bullwhip. The iconic whipcrack sound is actually the sound of part of the whip accelerating to the speed of sound and creating a miniature sonic boom, suggesting the tail blade may reach the speed of sound. This level of speed along with high precision is implied in Book 10 - The Android when Marco says that the Ax could slice a human into "bite-sized chunks" in "2 seconds."

The Tail Blade isn't just fast but extremely powerful. Ax often defeats enemies non-fatally by hitting them in the head with the flat of his blade, knocking them out, but if need be it can be extremely potent slashing weapon.. On the low end, Ax estimates that he would be able to slice a human in half with one or two swipes in Book 38 - The Arrival. However far more impressive than fighting humans, Visser Three consistently shows the ability to kill his subordinates in one blow, including Hork-Bajir, the shock troops of the Yeerk Empire. Ax himself has also shown the ability to casually slice off the arm of a Hork-Bajir in Book 5 - The Predator. Hork-Bajir are a particularly strong alien race, tough enough that in Book 13 - The Change the Hork-Bajir Ket Halpak is slammed into by a truck and is not particularly injured, being up and running almost immediately after. These Hork-Bajir are used to fight the Animorphs' battle morphs, a Wolf, a Gorilla, A Siberian Tiger, and a Grizzly Bear, in melee combat and present at least a moderate threat, with the latter two species being able to easily produce several thousand joules of energy. Ax doesn't morph to fight as he is consistently treated as equal to or superior to the other Animorphs' battle morphs. For instance in Book 12 - The Reaction, he decapitates a crocodile in one slice that Rachel, the gung-ho optimistic Animorph who was in Grizzly Bear morph, thought was an unstoppable tank. Even more impressive than this, in Megamorphs 2 - In the Time of Dinosaurs Ax kills a Tyrannosaurus Rex with three blows of his tail, with Ax concluding afterward that in a fair fight, he would be able to defeat a Tyrannosaurus Rex "only" 10% of the time despite the immense size and weight difference. There are also numerous feats of the Andalite tail blade cutting through metal and even steel. In both Book 15 - The Escape and Megamorphs 4 - Back to Before Ax cuts through steel, a steel door in the former and a steel net in the latter. He also destroys cages that are likely steel in Book 52 - The Sacrifice. Outside of Ax, another Andalite known as Gafinilan tears through a steel door in Book 40 - The Other. Gafinilan is a powerful Andalite, even larger than Alloran, but is dying from sickness, and the purpose of the scene is to show his strength even in his much-diminished state which Visser Three should scale far in excess of. It's fair to say that through scaling above the likes of a Tiger and a Bear, decapitating a crocodile that could tank their attacks, killing a T-Rex, and slicing through steel, Ax's tail should be wall level in potency, with Visser Three being a larger and more powerful Andalite. 

As mentioned above, Slappy has a pretty unfortunate tendency to be physically contested against by normal human children, but this is clearly just done for the purposes of allowing there to be conflict and agency on the part of the protagonists. In Slappy's Nightmare Slappy is explicitly stated to have "inhuman strength" explained as being "the strength of all his evil." Immediately following this Slappy begins to pull off Jimmy's head, Jimmy being a normal human, though is interrupted. The strength to pull of a human's head is generally regarded as requiring thousands of joules of energy. While technically speaking the events of this book are a dream sequence, these events are meant to be treated as credible and the feat's actually on the lower end of Slappy's notable feats. In Slappy New Year Slappy withstands going through a garbage compactor, a machine that crushes materials with a power of around 2000 PSI, a force of roughly 136 times the pressure of the Earth's atmosphere, a force that can crush cars, though he is described as limping and staggering afterward. In I am Slappy's Evil Twin, Slappy battles his twin Snappy, the two hurting each other with blows and treated as equals, with Slappy eventually ending the fight by slamming Snappy into a propane tank which creates an explosion that destroys most of a garage, catapulting both Dummies. At the end of the book, Slappy is revealed to have survived, though his condition is unknown. However, Snappy was not seriously injured from the explosion. In The Dummy Meets the Mummy!, Slappy is revived to battle the mummy Arragotus. Arragotus is powerful enough that with two blows he crumbled a large dinosaur skeleton to pieces, and smashed through the stone bottom of his sarcophagus. Yet Slappy was able to completely tank his attacks, Arragotus using Slappy "like a punching bag" yet unable to do any damage to him as Slappy continues to berate him. For being able to tank stone-shattering blows, scaling above an explosion that destroyed most of a garage, and surviving the inside of a trash compactor, Slappy should most likely be considered wall level in terms of physical strength and durability. There is an argument Slappy is significantly stronger as he is consistently hyped up as one of the most terrifying threats in the Goosebumps universe however this may not refer to Slappy's raw power but instead to his array of abilities or just his general ability to incite fear into others and scaling him to something like King Jellyjam in raw power is dramatically less consistent. 

Slappy's speed is much less clear, though he does consistently seem to be an agile and dexterous character, often avoiding strikes from normal people. According to the Goosebumps "Splat Stats", Slappy has a 6/10 in speed, the same as the Purple Rage. The Purple Rage is the arch-enemy of Dr. Maniac from Dr. Maniac vs Robert Schwartz, Dr. Maniac being a villain who is depicted flying between buildings quickly, requiring likely superhuman speeds, with The Purple Rage being his equal. Similarly, a very common monster type in Goosebumps are Werewolves, who likely can move at similar speeds to normal wolves, between 13.8 and 16.5 meters per second, which is again, slightly superhuman. Slappy is probably slightly superhuman in speed by scaling to them.

Slappy has numerous other physical abilities even before getting into Slappy's magic, though with some of Slappy's abilities it is unclear whether they are magical or not. As might be expected of a living ventriloquist dummy, Slappy is biologically immortal and does not age nor require biological conditions like sleeping and eating to persist. Slappy's body is also universally animate. In Slappy Birthday to You, Slappy demonstrated the ability to act as normal even after decapitation and he also showed the ability to move his severed limbs independently of each other. Slappy has a healing factor that seems very thorough but also seems to take significant time to work as in Bride of the Living Dummy he was cut in half with a sawblade and returned without comment in the next book. He also demonstrated the ability to make his body weigh significantly more than normal in Revenge of the Living Dummy so a normal human couldn't pick him up. Slappy's "blood" is a green slime-like material that is considered boiling hot. In Slappy Birthday to You he demonstrates the ability to fire it as a projectile from his nose and in Bride of the Living Dummy, Slappy demonstrates the ability to fire it as vomit from his mouth. 

There are some other abilities that might be "biological" abilities of Slappy's. In The Dummy Meets the Mummy! Slappy demonstrates the ability to shoot a red laser from his eyes capable of halting Arragotus and was able to spew out a swarm of cockroaches in Diary of a Dummy. In Slappy's Nightmare, Slappy demonstrates the ability to create a painful whistle sound compared to the pain of having a knife in one's ears. Finally, Slappy has a natural supernatural level of malevolence, created by the wizard Ephraim Darkwell for the purposes of enacting his evil vengeance, often hyped up as one of the most evil villains in the series. Slappy's malevolence is the source of his natural power and seems to provide him his resistance to a variety of abilities. Slappy's malevolence also gives Slappy his strongest ability not including his magic: even if Slappy's body is destroyed, his evil can possess a nearby living thing as demonstrated in Bride of the Living Dummy. 

Mental Capacities:

While Visser Three regularly gets outwit by the teenage Animorphs, this is clearly a narrative conceit to allow the protagonists to have agency in the story, combined with Visser Three's personality weaknesses. Befitting his status and feats, Visser Three should be an extremely intelligent character. Esplin, through his cunning, quickly rose to the status of Visser Three, the third highest position in the Yeerk Military, and eventually even reached the status of Visser One, the highest position. In The Andalite Chronicles, through his cunning he managed to deceive the Andalite Elfangor-Sirinial-Shamtul and steal the body of Alloran-Semitur-Corrass, the taking of an Andalite body being an unprecedented feat in Yeerk history for which he is called "The Abomination" by the Andalites. In The Hork-Bajir Chronicles, Esplin is depicted as having greater foresight than the other Yeerks, using his precious little hosted time to learn about their enemies the Andalites so that he might become invaluable when that information is needed.  He has personally fought in many of the biggest military campaigns of the Yeerk Empire, including the war for the Hork-Bajir, and created a new military division of Blue Band Hork-Bajir, an elite squad. 

However, all of this is only a shadow of Esplin's greatest mental advantage. Yeerks are able to absorb any information they want to from their host body, knowing exactly what they would do in any situation. War Prince Alloran is of course an extremely skilled and experienced Andalite, befitting his military title of War Prince. However more notable than this is the second of the three great weapons of the Andalites; their sheer intelligence. Andalites are somewhat known for their elitist arrogant culture, known for treating other species as hopelessly primitive because they are an advanced alien race of supergenius intellects. This is most notable with Ax of the Animorphs who, despite it being a running gag that he didn't pay much attention in his classes back on the Andalite Homeworld, more interested in Andalite females than his studies, regularly considers the best human science as primitive and demonstrates superhuman displays of intelligence. In Book 49 - The Diversion, Ax shows he can hack into the CIA for fun, in Book 46 - The Deception Ax creates a program able to "crack any possible security code conceived by humans",  and in Book 35 - The Proposal Ax disarms a high-security burglar alarm while in squirrel morph. In Megamorphs 2 - In the Time of Dinosaurs Ax says that fusion technology like those used in Fusion Bombs is like those used in the toys of children on the Andalite Homeworld in and in Book 8 - The Alien Ax accidentally advances human science by a century or more, by his estimation, after he accidentally corrects the errors in a scientist's software, assuming from its simplicity that it was a simple game for children to find and correct the errors. In Book 45 - The Revelation Ax develops a Z-space transponder able to monitor Yeerk communications and send transmissions to the Andalite Homeworld in a distant star system using Z-Space, an infinite alternate dimension used for FTL travel in the Animorphs universe. In Book 43 - The Test Ax modified a Dracon Beam, one of the Yeerk's weapons to not fire beyond a setting three and in Book 42 - The Journey Ax has what I believe to be his most impressive intelligence feat. Ax modifies the ship of a Helmacron, a tiny species of aliens the size of fleas in order to turn it into a shrink ray, using surgical equipment to manipulate the tiny machinery of the ship with parts the size of human hairs. Visser Three doesn't show that many intelligence feats himself, but should scale to Ax in intelligence through absorbing Alloran's information, especially given Ax's records as a bad student and how casual most of these feats are, with the last feat for instance Ax commenting that "the device is very simple to operate." By the intelligence scaling standards I use, this means Visser Three should be classified as a Type 2 Superhuman intelligence scaling from Ax being able to manipulate Z-Space to send inter-dimensional messages, modifying Type 2 civilization technology quickly, and manipulating Helmacron Technology to make a shrink ray.

Andalites also have a few other mental abilities derived from their extreme intelligence. Most famously while Andalites don't have mouths, they communicate with each other and other species via "thought-speak," a form of telepathic communication where the user transmits thoughts and ideas directly between minds. All species can thought-speak while morphed, but Andalites are one of the few that can do so in their normal state. In Megamorphs 2 - In the Time of Dinosaurs it is confirmed that all species can understand thought-speak because it "works at a level beyond mere words." Andalites also have an extremely potent sense of space and time, which Ax usually uses to inform the other Animorphs how long they have been in morph and how close they are to the two-hour time limit. Ax's most important feats of this ability come from Megamorphs 2 - In the Time of Dinosaurs where he can sense the rotation of the planet, sense its altered sense of rotation in the Jurassic Period, and that a nearby comet is not on trajectory to hit the Earth. It is possibly through their greater space-time awareness that Andalites have enhanced reaction speeds. Despite the blinding speeds of their tail blades, Andalites are known to duel with their tail blades, blocking and intercepting each others' strikes mid-strike. This level of reaction speed is similar to Megamorphs 3 - Elfangor's Secret where Ax dodges bullets multiple times, albeit from earlier time periods, the latter of which being from the 1940s. Similarly, in The Andalite Chronicles the Andalite Elfangor performs the seemingly unthinkable feat of piloting a spaceship moving at explicitly supersonic speeds near the surface of the Taxon Homeworld while attempting to evade a Yeerk Bug Fighter. There is technically an argument for their reaction speeds being far higher. In Book 52 - The Sacrifice Ax notes that Cassie "released her grip on the passenger bar a nanosecond before the blue-and-white streak sizzled past her." If Ax can truly perceive events happening in the nanosecond range, he would have FTL reactions, however, this level of reaction time is vastly inconsistent with anything else in the series and isn't one I would use. I include only for the sake of completion. 

Slappy has survived since the 1800s and his picked up a wide array of skills and a broad base of intelligence over the years. Slappy, despite his small stature, has become a leader of Horrorland, even masterminding the demise of the prior leader The Menace by belittling him, removing his source of power. Slappy is considered one of the smartest villains in the verse with it being stated that "Slappy is one of the smartest, meanest villains to ever set tiny wooden feet in Horrorland" and with him being rated a 9 out of 10 in Intelligence in the Splat Stats. This could allow for potentially insane intelligence scaling as the Horrors, a normal race of Monsters that work at Horrorland and which are clearly inferior to Slappy in intelligence can create such attractions as the Doom Slide which is an infinite space. However, it's unknown to what degree magic was used to create these inventions as many dangerous items in the Goosebumps universe are not purely scientific in nature but are devices cursed by magic such as the Evil Camera. However, this does suggest Slappy might scale to being in the same range of intelligence as Dr. Maniac, a supergenius-type evil inventor who is a recurring character in Horrorland, and who created technology to access an alternate dimension called Cyberspace. Slappy in particular specializes in psychological warfare, finding his victims' insecurities and tormenting them through that, either by framing them for things they didn't do or insulting them on things they're sensitive about, something he can often figure out in seconds. It is said Slappy's nasty insults have the power to ruin anyone's day as part of a Horrorland Dossier on Slappy, supporting him being top tier in psychological combat in the verse. 

Slappy demonstrates numerous types of specific skills as well, likely acquired over his long life span. Slappy has shown some basic melee combat proficiency in The Bride of the Living Dummy when he battles two humans, treated as his physical equals, at once and in I am Slappy's Evil Twin where he fights his twin Snappy, a complete equal of himself, and manages to get the advantage long enough to throw his twin into a propane tank. In Diary of a Dummy Slappy shows an extreme ability to speed read, flipping through the dummy Goldie's spellbook at a rapid pace and memorizing the spells just from the momentary glances at the pages. Slappy in general has shown an extreme skill at magic. In Slappy, Beware! The Sorcerer Ephraim Darkwell, Slappy's creator, granted Slappy his magic and almost immediately afterward Slappy impresses the sorcerer via his magical skills and he grows to the point that he considers Darkwell a bad sorcerer in comparison. Slappy is also highly skilled in stealth, with infiltration tactics being a particular favorite of his and it being stated in Night of the Living Dummy II that Slappy moves "silently as a shadow." 

Also, it's unclear whether this is a magical ability or a psychic ability, but Slappy has the ability to brainwash people into becoming his slaves. In Son of Slappy, Slappy's mind is stated to be able to produce a "signal" with his brain that can mind control others. This power allowed him to make Jackson Stander who normally is a "goodie-goodie" morally upstanding person think that ruining other people's posters is "awesome." 

Morphing:

Because of Visser Three's Andalite body he has access to the "crown jewel" of Andalite technology, one that is within his very person, and the third of the three great Andalite weapons, the power to morph. Morphing is the process of touching a living being to "acquire" its DNA and then being able to take on that creature's form and gain its abilities as well as all the instincts it possesses. It is the Animorphs' greatest weapon and for the majority of the series, Visser Three is the only morph-capable Yeerk making him something of a dark reflection of their powers. But while they have access to the animal species of Earth, Visser Three has access to some of the most dangerous predators of the galaxy. For a novice, morphing takes several minutes but for very experienced Morphers the time decreases with numerous instances of it being done in under 30 seconds. Visser Three, in a fight for his life in Book 48 - The Return, demonstrates the ability to morph almost instantly mid-fight. Visser Three's Morphs include:

Djabala: A small tree-climbing creature of the Andalite homeworld which is the first animal all Andalites acquire for training purposes, including Alloran

Kafit Bird: A twelve-winged bird native to the Andalite Homeworld Visser Three can use to fly and track down opponents. 

Taxxon: Taxxons are a humanoid-sized centipede-like race known for their insatiable hunger strong enough to overwhelm even a Yeerk trying to control them or an Andalite in morph as one, so strong that if a Taxxon is injured all Taxxons nearby will start cannibalistically devouring them including the injured Taxxon itself. They are a slave race to the Yeerks who promised them food in exchange for servitude and Alloran acquired one on a mission to the Taxxon Homeworld. They are not very good at fighting but are extremely good at working technology due to their many limbs, can breathe underwater and swim as fast as a Dolphin, and are extremely potent diggers, far better than anything on Earth. 

Hork-Bajir: Hork-Bajir are the shock troops of the Yeerk Empire, a species of peaceful dimwitted tree-herders with a lot of natural power and weapons, Hork-Bajir are large aliens with many sharp spikes on their body. They have an extreme ability to climb trees and with their raw strength and durability are considered a threat to the Animorphs' battle morphs including a Siberian Tiger and a Grizzly Bear. They have an exceptionally high endurance to pain. In Book 13 - The Change the Hork-Bajir Ket Halpak withstands being hit by a truck without much injury. And in the same book, to prove he's not infested by a Yeerk, Ket slices off part of his head to expose his brain and demonstrate the lack of any Yeerk with seemingly no pain. Alloran acquired a Hork-Bajir morph that Visser Three should have access to.

Antarean Bogg: In Book 1 - The Invasion Visser Three morphed an immense tentacled monster known as an Antarean Bogg to consume the Andalite Hero Elfangor. Its two legs are described as being the thickness of redwood trees and it uses tentacles to grab prey and consume them.

Unnamed: In Both Book 1 - The Invasion and Book 47 - The Resistance, Visser Three morphs an unnamed regenerating creature with eight heads, eight clawed arms, and eight legs, a very large creature that has the ability to shoot fireballs from its mouth. 

Vanarx: The Yeerkbane, the only natural predator of Yeerks on their homeworld, the Vanarx is a creature with a body like a raptor and a long purple tube mouth. It can latch its head to a controller and suck the Yeerk out, a tactic Visser Three uses to kill a Yeerk that has failed him without harming a particularly valuable host. 

Unnamed: An unnamed creature from Book 2 - The Visitor, Visser Three morphed into a stone-like creature described as being over 15 tall long (~4.57 meters), strong enough to throw around a bulldozer and crash through stone without injury. 

Mardrut: A Giant sea creature that lives in the oceans on one of the moons of the Andalite Homeworld, the Mardrut is a whale-sized predatory creature capable of swallowing a shark whole and swimming at roughly 20 MPH (~9 meters per second)

Lerdethak: One of the creatures the Arn, supergenius biological engineers, created on the Hork-Bajir homeworld to keep the Hork-Bajir from venturing too far into the deep. It is said to stand over 30 feet tall (9.144 meters tall) with poisonous razor-sharp tendrils and the ability to paralyze species with its mouth as well the ability to camouflage itself as normal plant life. 

Lebtin Javelin Fish: A large aquatic morph resembling a stingray that can shoot out biological spears that can paralyze prey animals, as well as out-maneuver dolphins and sharks. 

Unnamed: Morphed in Book 15 - The Escape Visser Three morphed into a serpentine creature that can breathe underwater, and is stated to be over 50 feet long (15.24 meters) and can swallow small boats whole, as well as having a poisonous spit. 

Unnamed: Morphed in Book 17 - The Underground, Visser Three changed into some flying creature with a 20-foot wingspan (~6.1 meters) covered in quills like a porcupine. This creature was tough enough to withstand a cave-in atop it.

Dule Fansa: A 7-foot-tall (~2.13 meters) creature with four spiked arms that can fire these spikes and regenerate them, with 4 shots destroying a house. 

Unknown: A lizard creature Visser Three morphed in Book 21 - The Threat

Kaftid: A horse-sized alien known for its fast movement and the ability to spray acid from its proboscis

Anteater: An Earth species Visser Three acquired for the purposes of fighting a species of microscopic aliens known as the Helmacrons

Unnamed: A strange floating extraterrestrial species Visser Three morphed in Book 29 - The Sickness resembling a floating eyeball whose tentacles paralyze those they touch

Unnamed: A giant crab-like alien Visser Three morphed in Book 30 -The Reunion capable of camouflaging itself to become essentially invisible as well as easily climb mountains or fight enemies with its giant claws

Luminar: An 8-foot-tall (~2.44 meters) alien from one of the moons of Sleegab Five that is perpetually on fire and which can both manipulate and project fire. The heat emanated from it can supposedly "incinerate anything it touches" though is obviously put out in water. 

Unnamed: A gigantic horrifically foul-smelling creature Visser Three morphs in Book 37 - The Weakness, this is an alien known for emanating a horrific stench and waves of heat, as well as being large enough to pick up a Polar Bear in one claw and towering over it with limbs the size of columns. It is also extremely resistant to conventional damage as Rachel in Polar Bear Morph charging it did no damage whatsoever to its "skin."

Unnamed: A similar morph Visser Three uses in Book 39 - The Hidden with a smell strong enough to knock a human unconscious and which is covered in acidic goop which it can throw as a projectile weapon. 

 Bievilerd: A flying alien Visser Three picked up on the planet Ondar, the Bievilerd is a flying predator species that snatches up prey in its wide mouth full of fangs. It is stated to fly "like a missile."

Unnamed: An unnamed reptillian gorilla-like alien creature standing over 30 feet (~9.144 meters) that Visser Three morphed during his battle for his life against Super Rachel in Book 48 - The Return. Given the context of the fight, it can be inferred this is one of Visser Three's strongest morphs as he selected it when forced to fight in a live-or-die battle by the godlike entity Crayak. The creature sweats acid through its' skin. 

Unnamed: A second blob-like alien Visser Three used in his fight with Super Rachel after she morphed into a giant alligator, its gelatinous form allowing him to escape her powerful bite by seeping from her mouth. It can pounce at an enemy and absorb them, digesting them inside it. 

Unnamed: An unnamed Octopus-like morph Visser Three morphs in Book 52 - The Sacrifice with twenty eyes and twenty eel-sized tentacles which he claims can tear the heads right off of people. 

Notably in this book, Visser Three morphs something to allow himself to escape or survive a huge number of bombs exploding in the Yeerk Pool, the equivalent of a small nuclear detonation, in less than 2 minutes. However, the method by which he did this is unknown.

Human: Finally, Visser Three has in many instances morphed several different humans for the purposes of infiltration.

Magic:

Slappy is known for his incredible magical prowess and the rapid rate he grows in skill at spellcasting. In Slappy Beware! Slappy grew in skill fast enough that soon after his creator Ephraim Darkwell granted him magic in the first place, Slappy was already impressing the old sorcerer. However, in Diary of a Dummy Slappy's magical powers grew even greater. Slappy had a sister even more proficient in magic than him named Goldie, whom Slappy sought the power of but who refused to share her magical power with her brother. In retaliation, Slappy tricked a human brother and sister into aiding him in stealing and reading Goldie's diary, absorbing the magic within and trapping Goldie in the form of a rat. With Goldie's magic absorbed, Slappy has gained a new mastery of magic and over time a wide variety of spells. 

Slappy's most used form of magic is biological manipulation. He can use this in a variety of ways:

Bodily Control: In My Best Friend Slappy, Slappy demonstrates the ability to use his magic to force people to mimic his movements. In his fight with his sister Goldie in Diary of a Dummy, Goldie bent and contorted Slappy's body into a pretzel and removed his mouth, spells Slappy should be able to replicate now that he has gained the knowledge from Goldie's diary. 

Animation: A fairly common ability for Slappy to use in combat, Slappy can cause inanimate objects to come to life to fight for him. In Slappy Birthday to You! Slappy animates an army of dummies to fight for him and in The Dummy Meets the Mummy! Slappy brings a dinosaur skeleton to life to battle the mummy Arragotus, this Dinosaur able to withstand a blow from Arragotus. 

Animal Manipulation: Along with his above-mentioned ability to control minds, Slappy has shown the ability to control swarms of animals in The Dummy Meets the Mummy! when Slappy commanded a swarm of insects to attack Arragotus, a threat the Mummy had even more difficulty with than the dinosaur skeleton.

Biological Corruption: In I am Slappy's Evil Twin, it is offhand mentioned that of the crimes Slappy committed, he caused cows to only make sour milk and a goat to be born with two heads. 

Plant Manipulation: Along with the above, it is mentioned that Slappy has caused the crop supply of the village to wither and die.

Size Manipulation: In My Friend Slappy, Slappy demonstrated a spell which he used to make stuffed dogs grow to the size of horses before popping, as well as gave them bared fangs somehow.

Sleep Manipulation/Dream Manipulation: In Slappy in Dreamland, Slappy demonstrated the ability to induce sleep in a group of people. He also shows the ability to explicitly control the dreams of others. 

Slappy has also shown numerous non-biological forms of magic.

Telekinesis: Slappy has shown on several instances telekinesis including telekinetically slamming a garage door down in Slappy Birthday to You!

Transmutation: Most famously Slappy turned his sister Goldie into a rat to trap her, and in The Streets of Panic Park, Slappy demonstrated a spell to turn people into dummies by pointing at them. 

Elemental Manipulation: In The Dummy Meets the Mummy! Slappy melted the amber encasing ancient insects so he could use them as an army against Arragotus. After reading her diary, Slappy should know Goldie's spells including creating streams of air compared to a hurricane, blazing heat like a fire, and creating living entities out of ice. 

Mirror Manipulation: Seemingly a relatively weak spell in the Goosebumps universe that most monsters can perform, in The Streets of Panic Park Slappy and the other monsters made their images in mirrors when they were not there.

Invisibility: In Say Cheese - And Die Screaming! Slappy demonstrates the ability to turn himself invisible to the naked eye though he still appears in photographs. 

Fourth Wall Awareness/Metafictional Perception and Potentially Manipulation: During the SlappyWorld series of novels, Slappy stops the narration to comment on the events to the reader, showing awareness of what's being written down as it's happening. During this time Slappy comments to the reader without anything happening and these interruptions are often ended with Slappy saying that the next scene will be at this place or time. This shows he has the ability to perceive metafictional things as well as to stop the events and possibly to shift the metafictional "camera." However, he hasn't shown much of an ability to use this in a combat particular way. As such, it's most likely the only real way Slappy can use this ability is to gather information about his current enemy/situation, and/or get time to think of a plan by halting the story. 

Weaknesses:

Visser Three still has all the same morphing limitations as the Animorphs do. He needs to lay his hand upon an organic creature to acquire its DNA and be able to morph it. After morphing an animal, especially if it's one he's inexperienced with, the instincts of that animal can overwhelm him, with the most powerful instincts being especially hard to fight. If he stays in the form of a species for two hours, he becomes a nothlit, trapped forever in that form, unable to demorph. He also can't go straight from one morph to another morph, he has to demorph to his Andalite form in between, and this process can be very exhausting (especially if done many times in a row), and even for an experienced morpher like Visser Three it does at least take SOME time to morph in which his body is more vulnerable than usual due to the randomly changing limbs and body structure.

Visser Three also has many psychological weaknesses. He is characteristically blood-thirsty, arrogant, and a show-off, regularly executing his inferiors for any perceived incompetence in theatrical ways. He wants to lord the incredible power of his Andalite body and morphs over his enemies. He is reckless and was the primary advocate of the Yeerks switching from a stealthy long-term infestation of humanity to an aggressive open invasion and taking the species by force. His own impulsive recklessness and arrogance are cited as the main reasons the Animorphs are mentally able to defeat him as often as they do.

Finally, Esplin has a couple weaknesses that come from his nature as a Yeerk possessing Alloran. Every three days he must return to the Kandrona pool to feed off the natural sustenance of the Yeerks, kandrona rays like those produced by their homeworld's Sun. Alloran is also an involuntary host, not a voluntary one. If given the option Alloran would vastly prefer to be dead than in Esplin's control and extreme acts of willpower on a host's part have allowed them to temporarily and partially fight a Yeerk's control, though this can only last seconds at most and Alloran's will has been so destroyed over the years it's unlikely it would be a large factor in most fights. 

Despite being a dummy, Slappy can be rendered unconscious and this is actually how he's usually defeated, either being reduced to unconsciousness by force or more commonly via the usage of his magic phrase. The phrase "Karru Marri Odonna Loma Molonu Karrano", Slappy's magic chant, is what awakens Slappy from his slumber, and hearing it again will also put him back to sleep. However, Slappy has found ways to get around this, generally by making it so he can't hear the phrase with the aid of things like earplugs.  

More than his physical weaknesses, Slappy has a lot of mental weaknesses. Slappy loves to toy with his enemies, preferring to scare and humiliate them rather than kill them, and only uses his arsenal somewhat randomly, seemingly based on how much of a threat they seem to be. He is arrogant and almost never believes there's even a chance of him losing, save against beings he knows very well to be stronger like Goldie. He is also a "control freak" and gets easily enraged whenever anyone even mildly resists his slightest demand. Although even when enraged he has sometimes avoided using his magic against normal humans, though these events took place primarily before he gained Goldie's magic so it's possible this is no longer a real weakness. 

The daughter of a famous celebrity gets into the news for her strange and erratic behavior, a result of her new dummy Slappy humiliating her and trying to turn her into his newest slave. Visser Three, hearing the news and thinking this girl may have word of the Yeerk Empire or the "Andalite Bandits" goes personally to capture the family at their remote house and infest them. Slappy however is not so fond of having his slaves stolen and the two villains end up in a face-off. Who would win?

In terms of direct stats, the two are surprisingly comparable in base. Physically any blow of Slappy's should dramatically injure the Visser if he can land one as the Visser's base durability is not especially impressive. However, Visser Three's tail blade is probably strong enough to do "damage" to Slappy and the Visser has dramatically higher attack and reaction speed. Mentally Visser Three is probably somewhat smarter and more skilled though Slappy can surprisingly compete in both categories, both being by my categorization a Type 2 intellect and a C Tier Fighter in skill, and has a more well-rounded base of skills overall. 

In terms of information immediately available to both combatants, Visser Three would be able to very quickly determine Slappy's inorganic makeup and his superhuman physical characteristics from his movements, and would likely conclude that the Dummy is some kind of small fighting automaton which is not.... TOTALLY off the mark. Though with no knowledge of the existence of magic, Esplin would lack knowledge on how Slappy works. In contrast, Slappy wouldn't be able to gain any immediate information on Visser Three, as an Andalite is totally unlike any creature seen in the Goosebumps-verse, though he should be able to make some obvious inferences like his tail blade is probably dangerous and his stalk eyes would make him hard to sneak up on. He may conclude that Visser Three is some kind of monster he doesn't know which would technically be true as aliens are considered a type of monster in Goosebumps. Due to his metafictional perception, Slappy would likely be able to gather some bits of information on the Visser as the fight goes on, such as what morphing is and the Visser's emotional state.

This fight hypothetically could end instantly if Slappy lunged for Visser Three and Visser Three just did what Ax usually did and hit Slappy on the side of the head with his tail blade, knocking the dummy unconscious, something that seems to follow the sometimes odd mechanics of both universes' fighting. However, this is not something Visser Three would do as he does not care at all for fighting non-lethally (plus has no sense that an automation even COULD be knocked unconscious) and would try in such a scenario to slice off Slappy's head. His tail blade is strong enough that might actually work....but it also would barely inconvenience Slappy as his limbs are all universally animate. While this wouldn't be too debilitating for Slappy, it would also make it very difficult for Slappy to try and engage Visser Three in melee since even in his base form, the Visser can just comedically knock Slappy away with the force of his tail blade. Even if Slappy adjusted his weight to make himself heavy, he's never made himself heavy enough to not get flung by that level of force.

Pretty soon after the fight starts, both sides would likely start using their more powerful abilities, in particular when it becomes clear that they can't easily kill the other with a casual physical attack. Visser Three is likely to use morphing first but as soon as he does, Slappy would recognize the greater threat Visser Three poses and respond with a subsequent escalation of power. So that brings up the obvious question, how well could each other handle the other's special abilities? How well could Slappy handle Visser Three's morphs and how well could Visser Three deal with Slappy's magic?

Starting with Visser Three's morphs, Visser Three's morphs obviously grant him a far higher degree of power, such that Slappy no longer would be any form of immediate physical threat and he could clearly incapacitate him with something like the Dule Falsa's spike projectiles. In addition two of Visser Threes'  morphs have shown the ability to use firey attacks which might be dangerous to Slappy, though Slappy in promotional art has shown the ability to resist fire. However, any of Visser Three's morphs that use biological forms of attack like paralysis or poison or noxious stench wouldn't do anything to Slappy due to Slappy's inorganic nature. While Visser Three's fire-wielding morphs might provide a threat, Slappy's ice-related magic may be able to put out a Luminar, something that would be fairly intuitive for him to think of. Visser Three's blob morph also probably would not work as its absorption is similar to that of Monster Blood, a substance in the Goosebumps universe Slappy is depicted as wielding without difficulty or risk of absorption. His camouflaging crab-alien morph would likely be easily countered if Visser Three uncharacteristically tried to use stealth as Slappy has metafictional perception that would allow him to just read how Visser Three is attempting to sneak up on him. There is one more threat, however, the Kaftid's acid, something Slappy has never really shown resistance to. So the major threat of Visser Three's morphs is that he might just overwhelm Slappy with raw power and knock him out, with a lesser threat coming from fire or acid attacks.

In contrast, how well would Visser Three be able to handle magic? Slappy's most common type of attack is biological manipulation, however for the most part this is unlikely to be very helpful against Visser Three. By morphing, Visser Three heals all conditions done to his body so long as the damage is not done to his DNA itself. Even if Slappy put Alloran's body to sleep, it wouldn't really do much as Esplin in control of his brain could just awaken him and unless Slappy learns about Esplin he wouldn't know to target Esplin himself for bodily control. You might think that Slappy would learn of Esplin from his metafictional awareness and while that's possible, I don't think that's very likely. When Visser Three appears in any novel, the fact that he is Esplin controlling Alloran's body rarely is mentioned unless it's plot-relevant. If Slappy at any point learns of Esplin he can just control the Yeerk and force him to leave Alloran but so long as that doesn't happen Esplin can somewhat counter most of Slappy's biological forms of attack. Similarly, Slappy's pain-inducing whistle is unlikely to work as Esplin can just...not absorb the information of the pain Alloran is feeling or the auditory information he is recieving, and animation may not work as Slappy's animation minions may not be strong enough to be relevant to one of Visser Three's battle morphs. This fact also gives Esplin a rather strange win con which is if Visser Three at any point kills Slappy and he tries to possess "Visser Three", if he ends up in Alloran's body, he would awaken under the control of Esplin. 

However biological magics are not Slappy's only magics. Slappy has the ability to summon hordes of creatures. For Visser Three's Andalite body, this would be extremely difficult to fight as his tail blade can't easily kill a horde of cockroaches, through the Luminar's flames or the Kaftid's acid could counter (or the anteater morph depending on the type of insects.) Slappy could also use transmutation and this would be a one-shot ability as soon as he used it. Even if it did only affect Alloran's body, Esplin would not be able to get him to change back and would be essentially out of options. As such the biggest threats from Slappy's side are transmuting Visser Three into a dummy or figuring out how Visser Three works and targeting Esplin himself with his magic. Slappy has another win condition as well. Slappy, as a dummy, has infinite stamina while Visser Three has very finite stamina he drains by morphing. If Slappy wears out Visser Three he can overwhelm him with anything, especially something like a swarm of insects. 

While both have options, Visser Three does have a pretty clear advantage here in the simplicity of his tactics. Slappy can counter all of Visser Three's side options but his main tactic of overwhelming an opponent with flashy raw power should work fine especially as Slappy is prone to using his magic somewhat at random and isn't especially a focused fighter. However, there is one factor that might keep Visser Three at bay. Visser Three's win conditions are all in the short term, it's brute force overwhelming the dummy before he can find the right spell or right information, or Visser Three knocking him out immediately. Slappy's are in the long term, figuring out Esplin's true nature, figuring out that the morphing alien can't return from being morphed against his will, or wearing Visser Three out of stamina. While it might seem Visser Three would recklessly throw himself into the battle, it's clear that Visser Three is not used to one on one fair fights, especially not things unlike anything he's ever seen, like a tiny magic-flinging automaton. Slappy is known more than anything else for his psychological ability to break down enemies and all the monsters in Goosebumps are fixated on instilling fear in their enemy. So really the central question here is would Visser Three be courageous enough to attack Slappy aggressively and beat him in the short term or would Slappy's mind games be able to hold the Visser off long enough to figure out the correct tactic?

Yeerks are not a predator race, they are a parasitic prey race, and prey races as shown often in Animorphs to be terrified constantly for their lives, with Yeerks being cautious and self-interested. In The Andalite Chronicles after first gaining Alloran's body, Visser Three is alone with the Andalite Elfangor, then just a youth Aristh (cadet.) Despite the obvious advantage in mass and experience, Visser Three holds back from engaging him because Alloran saw Elfangor's talent at tail-fighting and Visser Three was scared at even the chance of dying. In a scene that seems to mirror this, Visser Three confronts Ax, Elfangor's younger brother, also an Aristh in Book 18 - The Decision , and once again holds back from confronting him despite the advantage in mass and experience. When victory does not seem very clear, Visser Three is not very courageous. In contrast, Slappy is regularly considered one of the scariest monsters in the Goosebumps universe, with some calling Slappy the scariest monster in Horrorland, scarier even than the Haunted Mask who can induce such terror as to make others go insane. Slappy also specializes in psychologically tearing down enemies, with being stated that Slappy's insults are potent enough to ruin anyone's day and with him gaining his leadership role in Horrorland mostly due to his ability to break down an enemy's will to fight. 

As such, while the fight would initially be a large danger to Slappy and he could get caught unawares, I think Slappy would be able to win the psychological mind game and break Esplin's resolve, keeping him from just overwhelming the dummy for too long until Slappy manages to figure out how to win, most likely through transmutation.  

Scenario 2:

What if Visser Three and Slappy both came armed for battle with all their optional equipment and with full intent to kill from the start?

While Visser Three doesn't typically use weapons, as most of his morphs can't effectively wield one, he does have a vehicle associated with him as well as could very easily get the standard weapon used by the Yeerk Empire, the Dracon Beam.

The Dracon Beam is a laser weapon that was adapted from the Andalite Shredder weapon. While the shredder is designed to kill as harmlessly as possible, the Dracon beams are designed for the opposite purpose, to purposefully inflict as much pain as possible in the process of killing enemies to suppress resistance, though they can be set to lower settings to avoid killing enemies. In The Andalite Chronicles Elfangor describes the Shredder's full power as being capable of blasting a hole through ten feet (3.048 meters) of solid metal alloy. The power of Dracon beams is described as being able to completely vaporize a person, a handy trait for a device used to quickly dispose of evidence. Vaporizing a person is generally thought to take hundreds of millions of joules of energy, or enough power to destroy a small building. While Visser Three doesn't typically use a Dracon Beam, if he was going to what he believed to be a difficult battle, he would likely bring one just in case. He may also arrive in his personal vehicle, the Blade Ship.

The Blade Ship is Visser Three's personal spacecraft whose design resembles that of a battle ax. It is described as being eight to ten times the length of the normal Yeerk "Bug Fighter", spacecrafts the size of school buses. It's described as moving a few thousand miles in seconds, requiring speeds easily in the massively hypersonic range, and is likely similar in speed to Andalite Vessels that, in The Andalite Chronicles, are described as being able to reach near the speed of light, though would suffer the effects of relativity from doing so. The Blade Ship is equipped with a larger version of the Dracon Beam, known as the Dracon Assault Cannon. In Book 28 - The Experiment Ax suggests that these weapons are capable of burning off the atmosphere of Earth, a feat that would require continental levels of energy. In Book 53 - The Answer Ax says that the Blade Ship would be a threat that most alien species could not fight, that is more dangerous than even the Pool Ship, the main battleships of the Yeerk Empire as it is powerfully armed, and far faster and more maneuverable, though seemingly lacking in defenses. Its defenses are not as strong but in The Hork-Bajir Chronicles, the Blade Ship's forcefields protect it from the ground forces assault of the Andalites and Hork-Bajir liking tanking numerous Dracon Beam blasts. Its durability may be far higher. Visser Three considers it unlikely in Book 53 - The Answer that blasts from the Pool Ship when drained to one-quarter power will harm the Blade Ship and the Pool Ship would have similar atmosphere-destroying capabilities. Though it's possible this is referring to speed rather than power as the Blade Ship casually dodges the Pool Ship's beams. The Blade Ship also contains an internal space capable of carrying hundreds of crew members, a torture chamber, as well as a small portable Yeerk Pool for them to feed every three days.  The Blade Ship does have a mild weakness that, unlike Bug Fighters, it is not amphibious and higher density surroundings like underwater keep it from functioning properly, requiring the Visser to build a special aquatic variant called the Sea Blade. 

While Slappy is the face of Goosebumps, he is really part of a trio of famed icons from the series, and Slappy has wielded both of the others as technology.

Monster Blood, or specifically the most common green variant of Monster Blood, is a slimelike substance in the Goosebumps universe known for its fast growth rate. While it can grow itself, if consumed Monster Blood causes the person who consumed it to grow at a similar fast rate until they are the size of a building at least as demonstrated in Monster Blood III. In the Stage Play Goosebumps Live on Stage: Screams in the Night Slappy showed that he had Monster Blood and used it to instantly grow in size, somehow bypassing the normal time it takes to grow giant via monster blood. While pure Monster Blood naturally attempts to absorb anything it comes into contact with, Slappy also showed that he can somehow resist this effect. What's more, Monster Blood seems to give animating malevolence to what is consumed by it. Evan's Dog Trigger after consuming the monster blood became more enraged than normal in the first Monster Blood and in Monster Blood for Breakfast! when monster blood is given to two dead ivy plants, they return to life and grow into dangerous monstrous plants, beginning to attack Bradley without him noticing at the end of the story. 

Slappy has also been known, on occasion to utilize the power of the Haunted Mask. The Haunted Mask is actually one of a group of Masks called "The Unloved", created by an Ugly Shopkeeper out of human flesh in an attempt to create a beautiful face, but with each face reflecting the ugly monstrosity of its creation. These Masks are alive and semi-possess the user, turning them more monstrous and feral. Along with the minor amplification of power that comes from animalistic rage, the main power of the Mask is its ability to induce extreme amounts of fear and horror in those who look at it, growing moreso as it slowly becomes the face of the person wearing it. The fear caused by the mask is so intense that in The Scream of the Haunted Mask, it is shown that horses die of fright just looking at the mask and a ghost flees the sight of a human wearing it, despite knowing the human can't even interact with them. The Haunted Mask resists any attempt to take it off, and can only be removed by "a symbol of love" though Slappy has presumably found some other way to remove it via his magic.  

Slappy has also been seen using a few other pieces of equipment on occasion including makeshift weapons, an electric joy buzzer and, most notably, in Revenge of the Living Dummy Slappy used the Mind-Stealer, a shrunken head with the power to steal the minds of anyone near to it. However, Slappy is not immune to the Mind-Stealer's power and was defeated when it stole his mind.

So what would happen in this scenario? 

It might seem pretty simple here that Visser Three flies in on the Blade Ship and fires atmosphere destroying blasts on Slappy's position and bombards him, getting victory by death. But is it that simple?

Maybe.

If Slappy showed up prepared to fight, presumably he would have already used the Monster Blood and that leads to the question, if Slappy died could Monster Blood revitalize him? Partly it depends on if Visser Three gets a direct hit on Slappy and just vaporizes the dummy body or if he blasts the surrounding area with the resulting flash heat burning Slappy's body. If it's the latter, Slappy's body could still be hypothetically within the range of being restored by Monster Blood's revitalizing properties. There's also the question of whether or not Monster Blood CAN revitalize an inanimate object like Slappy. Personally, my guess would be that it could because it has been shown to make him grow before and when Evan consumed Monster Blood in Monster Blood II it didn't just make Evan grow but also his clothes, suggesting that it works on inanimate material as well. 

Assuming Slappy does indeed survive the initial assault, how could he counter-attack? Slappy's magic should work just normally, ridding Visser Three of his technology by transmuting it (though in fairness, he's never shown the ability to transmute something as big as the Blade Ship.) In this scenario, Slappy would also have access to the fear inducement and mind stealing of the Haunted Mask and the Mind-Stealer. The big difficulty Slappy would have in this scenario is range. While it's not really in character for Visser Three to bombard enemies from orbital range, The Blade Ship would probably let Visser Three attack Slappy from extreme ranges, comparable to the tens of thousands of kilometers that Fighter Jets attack from, especially combined with Visser Three's superhuman sense of spacetime. In contrast, the range of most of Slappy's magical abilities is unknown and has never shown a range greater than several meters. Slappy could somewhat mitigate this obviously by growing to building-sized with Monster Blood which presumably would amplify the range of his abilities but not only would it still likely be lower, but this is essentially just making himself a giant target.

The best chance Slappy has for this matchup is actually using the Monster Blood and Haunted Mask in combination to grow giant and project his horrifying visage to break the Visser's will when he sees Slappy on screen from a large distance away, and making him retreat from the fight. While this is somewhat of a credible scenario, I think Visser Three would probably win Scenario Two just due to overwhelming power and range. 

Scenario 3:

Both Visser Three and Slappy have arguably attained a vastly higher level of power than their norm though in both cases it's somewhat arguable whether it should apply to the character. You can view this final scenario as the two fighters at their respective peaks, giving the full benefit of the doubt. 

In The Andalite Chronicles Visser Three very temporarily had control over the Time Matrix, though he was fighting for control of it with Elfangor the entire time, but in this benefit-of-the-doubt peak scenario he would have it. The Time Matrix is the strongest weapon in Galactic History, stated to be the strongest weapon to have ever been made. If the user thinks of any place in spacetime, the Time Matrix brings them there, taking them through space and time alike to bring them there. If they imagine something that doesn't exist the Time Matrix will create a new reality with that thing existing, and if multiple try to control it at once, it will create a warped new universe comprised of both locations that were being imagined. It can also be used to control the minds of people such as when Loren in The Andalite Chronicles used it to return to Earth with no one questioning her difference in age. The Animorphs universe is actually used as part of a giant game between two universal reality-warping entities called the Ellimist and Crayak, higher dimensional entities. The Ellimist actually created the Time Matrix, a four-dimensional reality-warping machine as a tool, and when Visser Four gained the Time Matrix in Megamorphs 3 - Elfangor's Secret, the two actually set aside their game to allow the Animorphs to pursue Visser Four in time, as what a three-dimensional entity might do with that power scared even them into working together. Its only limitation is what can be imagined, meaning that with access to the Time Matrix, Visser Three would gain hypothetically time travel, teleportation, universal reality-warping and creation as well as higher dimensional manipulation.

Beyond that Scholastic owns the rights to both Animorphs and Goosebumps and has not made an official policy of canonicity clear. If it's accepted that other Animorphs products are canon to Visser Three, then in the video game Animorphs: Shattered Reality, Visser Three invents a machine that he uses alongside a "Continuum Crystal" which literally shatters reality into pieces. In an official toy description, Visser Three is also noted as having an inferno morph with enough power to "bring the entire universe under his control"

As mentioned above Scholastic has not made clear an official canon policy for both series. In the Goosebumps film series, Slappy, who is very strongly suggested to be the same as the one from the book series, is released from his books. An official film description states that Slappy is RL Stine's evil half (something heavily implied in the films themselves,) RL Stine being a writer with the strange ability to create real monsters, the monsters of the Goosebumps universe. Slappy gains control over RL Stine's manuscripts and the ability to bring the monsters to life, the Goosebumps monsters inherently viewing Slappy as their leader, likely due to being the part of RL Stine that brought them to life. Via this, Slappy has access to hypothetically every ability possessed by monsters in the Goosebumps universe, if only because he can summon them to his aid.

This would include the power of the Verona XG-20, a universal remote from the Tales to Give You Goosebumps short story "Click", a remote that can control the universe as though it was a remote controlling a tv show including turning the universe off with the power button. This also includes the Existence Erasure of the Cursed Camera in Say Cheese and Die! Clarisa's Wish-Granting Reality-Warping from Be Careful What You Wish For, The Masked Mutant's Metafictional Movement able to exit from a comic book world in Attack of the Mutant, The Grool's Probability Manipulation from It Came from Beneath the Sink!, Rip the Cat's Lifeforce Manipulation and 9 Lives from Cry of the Cat, The Body Squeezer's possession and time travel from Invasion of the Body Squeezers, Emmy the Rogue AI's technopathy from Don't Scream, Gargoyle Slime's petrification from the short story "I'm Telling" from Even More Tales to Give you Goosebumps, as well as ghostly abilities from a LOT of ghosts who tend to have invisibility, intangibility, and possession. 

As seems to be the case in every scenario involving these two this scenario is a glass cannon battle where both have such overwhelming offenses compared to their defenses that either can win basically anytime. Slappy has more ways of killing Visser Three here but the question here is more a matter of who would get the first hit on each other when both are given reality-warping abilities this strong. 

Obviously, both fighters here have time manipulation abilities with the Time Matrix's primary function being to take one anywhere in spacetime and Slappy having the Verona XG-20 which can "rewind" time like a remote controller. However, both fighters can avoid being wiped from the timeline from their opponent attacking their history. For Visser Three, the Time Matrix is a higher-dimensional object that can protect the user by keeping them outside the normal temporal dimension. Conversely, while the Verona XG-20 wouldn't inherently protect Slappy from having his timeline altered, he can use the metafictional movement of the Masked Mutant to escape the fictional reality he and Visser Three are fighting from, making it impossible for Visser Three's changes to affect him. Similarly, even if Visser Three segments reality with the Continuum Crystal Slappy could simply go around the division between reality segments by "exiting" into a metafictionally higher reality and then re-entering the story wherever Visser Three is.

That leads into Slappy's biggest advantage in this scenario which is his ability to gain information. Neither Visser Three or Slappy naturally have the ability to perceive four-dimensionally, so the moment Visser Three or Slappy begin time traveling, they would have no idea where each other is in the timeline the whole time and will by default only show up in the same time and close enough in space to find the other by sheer chance. However, Slappy can perceive metafictional narration and could likely "read" where Visser Three travels with the Time Matrix or simply leave the story and read where he went as though it was in a book. 

Beyond this Slappy has a few more advantages. While neither has very strong defenses, Slappy does still have more than Visser Three. He is still a soul possessing a dummy, he can hypothetically gain access to the probability manipulation of the Grool or Rip the Cat's 9 lives. These probably won't last very long against a concentrated assault by someone as powerful as Visser Three would be with unchallenged control of the Time Matrix. However, especially given that Slappy is like nothing that exists in the Animorphs-universe, it might save him a few times as Visser Three focuses on trying to destroy him via conventional physical means. Meanwhile, Visser has essentially no such defense and a simple trick like Slappy showing up where Visser Three and is turning off the universe would probably one-shot Visser Three.

Now it's not entirely one-sided. Visser Three has greater reaction speed and is a more serious focused fighter than Slappy and if the two came down to a quickdraw situation, I would bet on Visser Three going for a lethal move with the Time Matrix before Slappy does with one of the Goosebumps Monsters. However, this one advantage is probably not big enough to circumvent Slappy's advantages and Slappy has one more advantage that basically guarantees he will win in the long run.

Visser Three's powers comes from an object, the Time Matrix, however anyone touching the Time Matrix when it's used takes part in controlling it and Visser Three in this scenario would depend pretty much entirely on technology for his abilities. Slappy can just as easily get this technology away either via destroying it, removing it from the Visser, or using Emmy to possess the Time Matrix. Conversely, Visser Three can't really remove Slappy's ability to summon the other monsters as it's inherent to his nature as the evil side of RL Stine. 

This scenario really sort of mirrors the first scenario. Visser Three can win early on due to both characters being a glass cannon-y type of character in the scenario, as well as having a more focused serious personality and faster reaction times. But in the long run Slappy's larger number of advantages and higher versatility means he's almost certain to win if the fight goes on past the initial scuffle.