The Mind Is a Plaything of the Body

""The awakened, the enlightened man says: I am body entirely, and nothing beside; and soul is only a word for something in the body. Your body is a great intelligence... Your little intelligence, brother, which you call "spirit", is also an instrument of your body, a little instrument and toy of your great intelligence.""

- Friedrich Nietzsche, Thus Spoke Zarathustra

Maybe it's just the animal instincts associated with your new form, a change in hormones or the complete rewiring of your neural circuitry, but for some reason the process of Shapeshifting tends to also transform the very way you perceive and think about the world around you.

This may be relatively minor (e.g., you notice smells more and food seems more exciting) or may be so extreme that you're not even recognizable as the same person any more. The latter is particularly tragic when a more drastic change is combined with a permanent transformation, as it can be seen as a form of death. It is also sometimes used to bless a voluntary shapeshifter with suck.

The title of this trope is a paraphrase of of Friedrich Nietzsche's declaration that the mind and soul are completely illusory products of one's physical brain and body (page quote).

Often part and parcel of the Super-Powered Evil Side and Transhuman Treachery. See Sense Freak for a much milder example, and Loss of Identity for the much more extreme end result. Cybernetics Eat Your Soul is a subtrope. Can lead to Immortal Immaturity and the Second Law of Gender Bending.

Compare Becoming the Mask, Shapeshifter Mode Lock and And I Must Scream; contrast Different for Girls. May lead to/from Sense Loss Sadness.

For the exact opposite effect, where the body is a plaything of the mind, see Your Mind Makes It Real and some forms of Empathic Shapeshifter.

Media in General

 * The classic Wolf Man fits this trope perfectly: a normal human most of the month but a rabid wolf/wolfman under a full moon.

Anime and Manga

 * Averted with Tanya from Saga of Tanya the Evil, who still identifies as a man after being reincarnated into the body of a girl.
 * Happens in Digimon Frontier when Takuya first attains his Vritramon/Burning Greymon form. He flips out and nearly takes Tomoki's head off, until one of Tomoki's tears falls into Takuya's eye, causing him to regain his sanity. While all of them had trouble controlling themselves in their beast transformations at first, only Takuya lost complete control. Except Izumi, for some reason; she attributes her ease of control to being a girl, but says it so obnoxiously that the boys feel ready to throw up. (Koichi also had no trouble, but he's the group's Sixth Ranger, and by the time he joined up the beast transformations were So Last Season.)
 * Not quite. he just got his beast spirit when whe was still Brainwashed and Crazy, and he became more Ax Crazy whilst using it then. He'd just got used to it by the time he turned good.
 * Averted in Digimon Tamers: a primary point was that the personalities of the Digimon in this series stayed the same; regardless of the previous Digimon rules. The English version tried to show this by having all the forms have the same voice; though some people didn't like the effect.
 * Interestingly, as Guilmon goes further up his evolutionary change in the dub, he actually seems more intelligent, even snarky at times. For example, while Guilmon was basically a five year old, and thus not really prone to insults, by comparison in the dub Wargrowlmon made comments like "Hold it hog breath!" and "Whaddaya expect, he's a pig!" when battling the pig Deva. He seems to revert to the more childlike mentality somewhat in the combined Mega form of Dukemon/Gallantmon, but this might be a side effect of merging with Takato. In Guilmon's case, the trope is played utterly straight when he is forced to evolve to the corrupted Mega form called Megidramon, which is a near mindless monster that nearly ripped apart the Digital World simply by existing!
 * Ryo's Digimon was an exception, but there's a reason for that.
 * Dragonball Z and it's predecessor gave us the 'Great Apes'. Control of the Great Ape form seems to depend on a number of factors, the individuals phisical strengh being one.
 * In Darker than Black, Body Snatcher Mao needs Phlebotinum to maintain a human level of consciousness; without it, his personality is eventually overriden by the instincts of the cat whose body he inhabits.
 * In Mahou Sensei Negima, Evangeline says that she acts much younger than her age because she's been stuck in a ten-year-old body for hundreds of years, and her personality is affected by that form. Also, when Sayo (the ghost of a 15-year-old girl) starts possessing a small doll, she starts acting much more light-hearted and childishly than before.
 * Although that might just be due to the fact that she's happy that she's not bound to the school anymore for the first time in 60 years, and actually has friends who are aware of her existence. Who wouldn't be lighthearted about that?
 * No one who uses the age-disguising pills ever acts differently unless it's a deliberate act to appear their "age." Ironically, it just makes Negi appear as old as he acts!
 * In a different kind of example, Albireo Imma's Shapeshifter Weapon allows him to become the person he transforms into: all the memories, actions, personality traits and instincts he observes of them are recorded into his many books (he has one for every person he's met). While he's subconsciously aware of his own presence, his body acts completely as the person would, so much that the person themself is essentially speaking. He's able to take over when he needs to, but for the most part the consciousness he imitates is in the driver's seat.
 * Also is Shiori's persona stealing ability which allows her to do largely the same thing, only for her she almost completely blanks out of consciousness to the point of thinking she's the person. However, she does leap instantly to Negi's defense
 * Shiori is apparently aware, behind the scenes, of what "Asuna" does. But not the other way around,.
 * In Ranma ½, the vast majority of the shapeshifting characters gradually (though not consciously) acclimate to their cursed forms, learning how best to take advantage of their traits (such as size, flight, dexterity, or appearance) and paying heed to their natural instincts. The most notorious example is with Ranma him/herself: although his personality and sexuality remain untouched, he grows considerably more comfortable with his female self, adopting feminine mannerisms and body language. Halfway through the series, (s)he thinks nothing of dressing up nice and going on dates with men (usually for some ulterior motive, but sometimes solely for the guy's benefit) even though the mere thought of wearing a skirt was anathema to him at the beginning of the story. But is it the body working on the mind, or just gradual desensitizing?
 * And in a Filler episode, Ranma gets knocked on the head while in girl form, making "her" believe she's really a girl (an extremely girly girl) who unfortunately changes into a boy. Of course, as this painfully stereotypical behavior is probably just a reflection of Ranma's own mental image of what "normal girls" are supposed to be like.
 * An extreme example is Miss Hinako Ninomiya: due to an unspecified illness when she was a little girl and Happosai's chi-tampering, her body was stuck with the appearance of a ten year-old, even though she's somewhere in her late twenties. She acts like an obnoxious, bratty, bossy, hyperactive child while in kid form, but when she absorbs enough Battle Aura to temporarily restore her adult body, she's much more aloof and calculating. She's still somewhat of a bully, though.
 * Another good example is Ranma's father Genma, who actually seems to enjoy being a panda more than being a human.
 * Then there's Rouge, who turns into an Asura - a three-headed, six-armed, fire-breathing demon-goddess. Normally, she's a kind-hearted Chinese Yamato Nadeshiko, but in her cursed form she's got a case of A God Am I, and her temper disappears. Her irritability is partially explained as a result of the agonizing back-aches caused by having three sets of shoulders. In contrast, Pantyhose Taro turns from bishonen to minotaur-freakshow, without the slightest change in his Jerkass personality, although he seems less calculating and more instinctive as a chimera.
 * In Pantyhose's case, he was baptised in the cursed spring soon after he was born. Changing from human to minotaur and back again for all your life is bound to result in a more consistent personality.
 * Former boy Hazumu Osaragi from Kashimashi: Girl Meets Girl typically responds "I really am a girl now" when her classmates remark on any changes in her behavior.
 * Probably averted since "he" was extremely feminine in his behavior even before the physical gender-shift.
 * Bagi, the Monster of Mighty Nature plays this for tragedy. Bagi starts out as a Petting Zoo Person, but turns more and more feral as time passes.
 * Inugami Hatsune, a member of Zettai Karen Children Esper team "The Hound," suffers from this. Her powers allow her to technically assume the form of an animal (typically a canine) but the longer she spends in a form the more animal-like she becomes. Generally, the only way to snap her out of it is to calm her down by feeding her. This is the task of her partner, Yadorigi Akira, whose mind-switch powers allow him to throw prey in front of her to catch (and also forces him to experience the feeling of prey being eaten each time).
 * Inverted in one chapter of Cat Eyed Boy. A Mad Scientist attempts brain transplants... but later discovers they can't be successfully performed, as the new mind housed in the body will eventually force its new body to match the one it was originally housed in.
 * The entire premise of Detective Conan relies on the aversion of this; for Shinichi and Shiho, their mind was what the local Fountain of Youth left untouched.
 * Makaku in Battle Angel Alita explicitly references this idea when he becomes quite intelligent after losing his initial large cyborg body and is reduced to an ambulatory head.
 * Used and subverted in The World God Only Knows. After a gender bending Freaky Friday Flip, the boy begins acting more feminine and gains an interest in dating sims for women. However, this is because the evil spirit inhabiting the girl's body is manipulating his emotions as the girl has no such problems while in his body.
 * Averted with 007 from Cyborg 009. He's the local Master of Disguise/Rubber Man and a classically trained actor whose powers can literally transform him into someone else... but once he dispells his covers, we see that his mindset hasn't been altered, and he's still good old G.B.

Comic Books

 * Snowbird, of Marvel Comics' Alpha Flight, runs the risk of Shapeshifter Mode Lock if she stays in animal form too long—because her mind gets subsumed by the animal. She once had to be talked down by Wolverine while she was an actual wolverine.
 * Doctor Manhattan has been drifting away from humanity ever since he got his new body, this may just be an extreme form of Sense Freak its never really made clear.
 * A combination of both seems most likely. Since he experiences all of his life simultaneously he becomes distant, but having nigh-limitless power and an indestructible body certainly changed things for him as well.
 * Jennifer Walters started out as a somewhat demure lawyer - her transformation into She Hulk initially made her far more aggressive and angry. Over the years though, her continued use of the She-Hulk form has meant that the effect of her powered up form is simply a far more uninhibited version of herself.

Fanfic

 * An unfortunate side-effect of Fauna's powers in DC Nation. Yes, she can do a partial shape-shift into most animals, as well as speak to and persuade animals to ally with her, but the harder she pushes her abilities, or the more wounded/angry she becomes in a battle situation, and the less she thinks like a human and the more she thinks like a beast. Worse, she remembers almost nothing of what happened during one of those "freak outs" afterward. As she told Changeling once, "You keep the mind of the mind of a man when you shift; I don't."
 * George in With Strings Attached. His ring allows him to transform into any animal he can imagine, but becoming that animal also causes him to gain the senses, instincts and tastes of that particular creature. A few times, he has to exert his own will to keep from doing something inappropriate, like eating something (or someone) he's not supposed to.

Film

 * In the My Favorite Martian movie, the alien polymorph gum can alter one's personality, depending on the alien. This comes up later, where the hero's girlfriend chews one, turns into a giant alien, and kills a couple MIB guards, even eating one. Afterward, she acts as if she had just been in a drunken party.
 * Subverted in the Remake of Day of the Dead and Land of the Dead: Bub is a vegetarian and thus doesn't want brains, and he follows orders due to being a marine and/or having a crush on Cross. In Land of the Dead, the gas station attendant zombie initially remembers that he's supposed to do something with the gas pumps. As the movie progresses, he regains more and more of his humanity, including compassion for his fellow zombies, tool and gun use, and even leadership.
 * In The Fly, the scientific genius Seth Brundle fuses with a fly, and as his body slowly begins to morph horribly, he realizes that, to his terror, his personality is being overridden by the instincts of the mindless insect. Arguably, the last we see of his original personality is when the crippled wreck that Brundle has become mutely begs for death.
 * Averted in Captain America: The First Avenger as even after post-procedure Steve Rogers still acts dorky and as Peggy notes, still cannot talk to women. In fact, this was why Erksine picked Steve and tells him as such.
 * To some degree, it was inverted with Schmidt who developed his ... unusual features in response to the serum because the serum hadn't been perfected yet and he was an evil man at heart. As its creator said, good becomes better, bad becoming worse.

Literature

 * In Piers Anthony's very first Xanth book, Magician Trent can change anyone into any animal. The form comes with instincts built in. Trent said something like (paraphrasing): "If you're a turtle, unless you have the instincts of a turtle, you couldn't survive as a turtle."
 * Later books introduced a character who could shape-shift but didn't get the instincts, making it a much more limited talent. This was due to her demonic heritage; it wasn't even her personal magic.
 * One of the laws of magic in Jim Butcher's The Dresden Files is to never change someone's form against his will: It will eventually destroy his mind, leaving just a normal animal. Changing your own form is allowed but still carries risks.
 * A good example of a first-person viewpoint to this trope is
 * Used specifically to avert Different for Girls in Jack Chalker's Downtiming the Night Side. Also applied to most of the transformations that show up throughout his work, particularly in the Well World series.
 * In The Belgariad, Belgarath pointed out that transforming into an eagle wouldn't necessarily help you travel faster, because you were likely to get sidetracked by every tasty animal below.
 * However, he did manage to resist Poledra as a wolf.
 * Barely.
 * He also mentioned that his mind was significantly more compatible with that of a wolf than that of an eagle, and thus he was able to think more clearly.
 * Note also that after Garion shifts for the first time, his thoughts become permanently more wolflike, and wolves consider him one of them. At one point he's challenged by a pack whose territory he has entered, when they would have just avoided humans.
 * In Robin Hobb's Farseer Trilogy, Fitz fakes his death by abandoning his body and binding his mind to Nighteyes's. When he is brought back into his body it takes him half a year to remember how to be human, and even then he is much more wolfish than before.
 * Shapeshifting in Ursula K. Le Guin's Earthsea Trilogy also works this way: The longer you stay in a given form, the more your mind gets taken over by that form's instincts. There are allegedly a number of mages who turned themselves permanently into dolphins this way; in the first book, Ged almost loses his mind when he's forced to flee over a long distance as a hawk.
 * One noteworthy legend in Earthsea is the story of a wizard who so enjoyed turning himself into a bear that he ultimately became the bear and killed his own son.
 * In H.P. Lovecraft's Cthulhu Mythos stories, the outer god Nyarlathotep is said to have a thousand forms. Each form is essentially a separate entity (it's implied that he can even manifest several in forms at once). As such, the behaviour and personality of different forms can be completely different.
 * This is only vaguely implied in the original works, but considering that Nyarlathotep's different forms functions essentially identically to the Avatars of Hindu Mythology, this is indeed probably the case, although a central intelligence connects them all in some sense.
 * In Oath of Gold, the third volume of Elizabeth Moon's The Deed of Paksenarrion, the Kuakgan talks about this with Paks, who was unable to continue her life as a paladin-in-training after being captured, tormented, and rescued, and who is at the brink of suicide when he finds her leaving her worldly goods in his grove's offering-bowl. She also hasn't eaten properly for quite a while, and has a lot of half-healed wounds. The analogy he draws between the mind and the body is that of the snail and the snail-shell; if you poke holes in the snail-shell, will the snail live?
 * In Garth Nix's The Old Kingdom trilogy, Lirael can make skins that allow her to take on the form of an otter, or a bear, or an owl, but each one alters her temperament for a while, even when she takes them off (the otter-skin gives her a great craving for fish, for example.)
 * Terry Pratchett's Discworld series plays it relatively straight. One notable example, and a reference back to it in a later book, likens the mind to water and the body to a jug or vase. The water is only water, it does not become the container, but it does take on the container's shape.
 * The longer Discworld werewolves spend in wolf form, the more they start to think like a wolf. The reverse is also true: They become more human the longer they spend in human form. They never entirely become one or the other.
 * The same is true of Borrowing: The longer you inhabit something else's brain, the more you start thinking like that something else. In Equal Rites, Esk nearly starves to death because she loses track of her human self, but doesn't have the proper instincts to feed herself as a falcon.
 * The Great God Om points out in Small Gods that the shape of the body influences the shape of the mind, so if you spend too long in the body of, say, a tortoise, you begin to think like a tortoise.
 * When the Auditors create human bodies for themselves, they eventually start feeling emotions. The end result is a mix of Auditor and human that has serious trouble being either.
 * The point of the 'water in a jug' analogy is 's realization that it is wrong; for a mind in a body, the 'water' is truly changed by the jug.
 * And then there's the bit where Granny Weatherwax Borrows the Unseen University, and thinks like a building upon coming out of it.
 * Addressed in a more technical form in The Science Of Discworld, in which the complicit relationship between physical brain processes, external experience, and minds are discussed.
 * In Animorphs, every morph came with its own set of animal instincts; these were mostly useful but some morphs such as an ant had instincts so strong they risked permanently overpowering the human personality.
 * Termites caused even greater problems, since they were entirely controlled by pheromones the bodies were hard-wired to obey.
 * This trope plays both ways in Harry Potter: If a person is involuntarily transfigured into an animal (and this includes werewolves), then they lose their human mind for the duration of the spell. However, with years of study, and at great risk, a wizard can train him/her self to transform into an animal at will and retain their human mind - mostly. (Wormtail suffered no ill effects from being a rat for twelve years straight, but Sirius sometimes turned into a dog to ameliorate the stress caused by Dementors.)
 * Sirius also once says that James had always joked he had a much sweeter disposition as a dog, implying that there are at least mild changes in personality/attitude which reflect the behavioral patterns of the animal they transform into. Of course this aspect is debatable because of the fact that the wizard doesn't choose which animal they transform into, they find out only once they've completed the training and see what they become (much like the Patronus charm), and implies that the animagus form reveals, in part, the true nature of the character in question. Even so, it is easy to see that a character like Sirius may have very playful and loyal dog-like nature, but frequently behave in harsh or unforgiving ways due to the emotional and psychological trauma of his human life, which might be less at the forefront of his mind because of the less linear nature of the dog's mind. It is also the way he escaped Azkaban without notice—the Dementors could not detect the less complex dog mind, and therefore didn't realize a human was leaving the prison.
 * Likewise, a Horcrux is Nigh Invulnerable because it's the inverse of a mortal—the spirit it contains will instantly die if it's destroyed, but the physical self regenerates.
 * The author also noted that animagi tend to suffer a sharp decrease in IQ when in animal form, presumably due to the shrinkage in brain size
 * In The Tenth Kingdom, Prince Wendell swaps bodies with a dog and gradually starts thinking like a dog, the dog also slowly starts thinking more like a human (but never quite gets it) although this may simply be training.
 * In Sherwood Smith's Wren to the Rescue, the protagonist is turned into a dog, and is explicitly warned that the longer she spends in that form, the more difficulty she will have retaining her human mind; she gets out of it with no more than a few transitory dog habits. In the sequel, her friend Tyron is forced to spend a significantly longer time as a dog and only retains his identity by extreme effort.
 * There's a several pages long passage in Three Men in a Boat, detailing how a person's mood depends entirely on what they are eating, and what this food is doing to the body.
 * In the Wild Cards shared world novels, edited by George R.R. Martin, Jeremiah Strauss is a shapechanger who turned himself into King Kong and then spent several years acting like a gorilla and trying to carry blonde women to the top of the Empire State Building. Later transformations into a werewolf from The Howling and the Creature from the Black Lagoon almost turned out badly as well.
 * In the Immortals Quartet by Tamora Pierce Daine finds that thanks to mind-speaking with and turning into animals, that she can no longer eat wild game. She can still eat beef, pork, chicken, and fish because she's never had an interest in inhabiting the minds or taking on the shapes of barnyard animals or fish.
 * At least for a while. By Realms of the Gods she can't even do that.
 * We're told in the "Five Hundred Kingdoms" series by Mercedes Lackey that only the most powerful Sorcerers or Sorceresses can transform a person into an animal without that person losing his mind to the animal. However, since the books focus on Godmothers, we don't see this in action. In the first book, Elena transforms Alexander into a donkey, but has to give him regular time as a human. In "The Snow Queen", Aleksia takes several animal forms, but even as a polar bear (which has a large enough brain to "fit" a human mind into) the animal instincts get stronger the longer she remains changed.
 * In Alastair Reynolds' The Prefect, one glitter belt polity consists of disembodied minds interacting entirely virtually, but recognizing that The Mind Is a Plaything of the Body, they maintain their human experience by becoming not brains in jars but instead brains-and-endocrine-systems in jars.
 * The skinchangers (who do not physically transform but can telepathically possess certain animals) in A Song of Ice and Fire have trouble remembering their human selves. When  is being taught to control his power, he is told to do certain things as a wolf to help keep his mind in control. He keeps forgetting to do these things, and as a result he acts largely on instinct while in wolf form. Some of the older, more experienced skinchangers seem to be able to use their human intelligence more effectively, however, though it is mentioned that certain forms increase the risk of losing touch with humanity. When a skinchanger's human body dies, they can possess their animals to keep on living, but eventually the animal forgets its human life and becomes an ordinary beast.

Live-Action TV

 * In the Star Trek: The Original Series episode "By Any Other Name", the Kelvans (huge emotionless hundred-armed aliens from the Andromeda galaxy) take human form. They eventually start thinking like humans, including having human emotions.
 * Similarly, members of Species 8472 suddenly become much more willing to negotiate after taking-on Human disguises in the Star Trek: Voyager episode "In the flesh."
 * In a reverse example, Ensign Lindsay Ballard is unable to reintegrate back into her life on Voyager after having been transformed into an alien.
 * Space Cases: In the Personality Swap episode, turning human doesn't grant Radu any new powers like the others, except that he automatically assumes the role of the leader and starts ordering the others, who follow him, just not entirely happily. Apparently, a superiority complex comes with human DNA, though it should come as no surprise since Harlan explained this while he was still human. Rosie's and Bova's personalities are also characteristic of their species and swap accordingly.
 * Quantum Leap saw a little bit of this at times, usually attributed to the "Swiss-Cheese Memory" side effect of Leaping. In one episode Sam leapt into a retarded man and exhibited some of those same traits in living his day-to-day life.
 * Buffy the Vampire Slayer: Giles in "A New Man"
 * The result of regeneration in Doctor Who. While deep traits and values are still a part of every incarnation, the personalities can be vastly different. Especially clearly shown in the first episodes of the Tenth and the Eleventh Doctors, where they spend time discovering their new selves. "No second chances; I'm that kind of man."
 * In The End of Time the Tenth Doctor says "Even if I change, it feels like dying. Everything I am dies. Some new man goes sauntering away. And I'm dead."
 * Of course, it's difficult to assess just how much of a real change happens, since even opinions on regeneration change from Doctor to Doctor. One seemed to think it was just like a job with 'replacements', Seven thought it was a change of perspective, Ten thought it was an actual death, and Eleven's comments suggest that they're like phases everyone goes through. You could argue that the way that different incarnations argue when they meet up proves just how very different they are, but then, maybe they argue because they're too similar.
 * The physical avatar that Harper builds for Andromeda is initially just a more solid presence for the ship's AI than the holograms she normally uses, but over time, dealing with humans and aliens on a more intimate and physical level, Rommy develops a more emotional and independent personality.

Newspaper Comics

 * One Story Arc in FoxTrot featured Jason turning into a miniature version of his own older sister Paige. To her disgust, Jason (now rechristened "Jasolyn") began acting more and more like a stereotypical teenage girl: flipping out over shoe sales and dancing to the "Backsync Boys." of course, this being FoxTrot, it turned out that the whole thing was All Just a Dream, so it leads to a bizarre sort of meta-example where the mind is a plaything of the body which is ultimately just a plaything of the subconscious mind...

Tabletop Games

 * Played straight with the spell Polymorph Other in the first and second editions of AD&D—aside from a chance of simply killing the target, its original personality could be subsumed by one more appropriate for the new form either instantly or over time. This is used as a plot point in the Dark Elf novel Exile, written in the second edition era, which features a sympathetic supporting character turned into monstrous form by an evil wizard and fighting a losing battle against the instincts that come with it.
 * Averted in Genius: The Transgression: shapeshifting yourself or others explicitly doesn't affect your mind, although turning someone into something that isn't alive will cause them to lose consciousness until it's reverted. If you really want you can use a powerful mind control device alongside your transforming device.
 * In GURPS: Magic shapeshifting slowly drains the IQ stat. If it drops to five (animal level) the transformation becomes permanent.
 * In Mage: The Ascension mages can shapeshift using the sphere of Life. However, if you do not have Mind magic, or the highest rank of Life magic, you eventually take on the mind of whatever form you have shifted into. You can also leave out the Mind magic and force others to change shape causing your enemy's intellect and self image to slowly melt away over the course of weeks to be replaced by an animal's.
 * In FreeMarket, this is given as the justification for why you have to play some sort of human, rather than a catgirl, a total conversion cyborg, or a robot with a human mind uploaded—putting a human mind in an inhuman body causes the mind to become inhuman—or, in more plain terms, the person goes insane.

Videogames

 * In the universe of Pokémon Mystery Dungeon, it's implied that this can happen when Pokemon evolve. In particular, a Lombre is unsure whether or not he wants to evolve into a Ludicolo, because Ludicolo are carefree Pokemon, and he isn't sure if he wants to be carefree.
 * This carried over into the anime as well. Notably Ash's Charmander, reliable and obedient evolved into grumpy and unresponsive Charmeleon, who further evolved into an even more disobedient Charizard. This is supposed to reflect how in the video games, a trainer can't fully control a traded Pokemon whose level is higher than the badge allows. For example, you can have a level 50 Charizard, but unless you get all the badges, it won't always listen to you. Charmander was, technically, a traded Pokemon. Although Charizard never followed his orders until Ash stayed up rubbing his hands raw to defrost Charizard after it was encased in ice.
 * In the anime, Dawn's Swinub/Piloswine/Mamoswine act exactly like Ash's Charmander/Charmeleon/Charizard, except it retains its obsession with Poffins.
 * A possession example: In the video game Geist, you can inhabit the body of various mice. While you retain control during those sections, your body is inherently attracted to the cheese in the nearby mousetraps, often to the point of affecting or overriding your control.
 * Also, the body's own preferences can prevent the player from going places. When the player possesses a woman in the shower, she refuses to exit into the hallway (guarded by a male soldier) until she puts on her clothes. And after the player attacks an engineer with a pair of robotic arms to scare him, he'll never go near them again.
 * In Metroid Prime 3: Corruption, the cyborg hunter Ghor is quiet and unassuming - until he plugs into his large mecha/PoweredArmor hybrid, instantly becoming gruff and hyperaggressive.
 * The reason given in Eternal Darkness for why Chattur'gha trumps Xel'lotath. (Necessary, since using the mind-over-matter philosophy would break the Elemental Rock-Paper-Scissors.)
 * In Sanitarium, the main character involuntarily changes forms throughout the game. Each form has a distinct personality, their own unique (fake) memories, and their own voice actor. Near the end of the game, he gains control over these transformations. During his first few voluntary shifts, he frantically reminds himself who he really is...as if afraid one of the other personalities will take over.
 * In Dragon Age Origins, the aptly named Mouse is a Mage Apprentice who shapeshifted into a mouse and can't change back in the Mage Origin. He actually talks about how his shape is well suited to hiding and sneaking and that's most of what he does nowadays as the reason why he can't change back.
 * In King's Quest VII, in the third chapter there is a lord who has been turned into a stag, and he says that the longer he is forced to stay in that form, the harder it becomes for him to use rational thought, and implores the protagonist to help remove the curse on the forest before he forgets himself completely.
 * Also played with in the Expanded Universe material for King's Quest III. The cat cookie spell eventually will rob the sentience from the person afflicted with it, trapping them in a feline's body forever. Yes, it is a Fate Worse Than Death, but considering Manannan inflicted it on his enemies several times and was going to kill Alexander as soon as his usefulness was up, this makes it closer to Hoist by His Own Petard.
 * Somewhat present in Ever 17.
 * Commonly theorized to be involved in Final Fantasy VII with Sephiroth. After he manages to turn the tables and take control of the Eldritch Abomination that was trying to Grand Theft Me him, he develops a sudden interest in devouring the Lifestream. The theory is, that's just how Jenova's hunger instincts work.
 * In Portal 2, GLaDOS's body has some built-in functions that affect any artificial intelligence connected to it.
 * In the first Ratchet and Clank game it doesn't matter what enemies are doing prior to being hit with the Morph-O-Ray; after it's turned them into chickens they just go about their chickeny lives.

Webcomics
"Kat: You should stay as a wolf more often, dude. You're much nicer this way. Reynardine: Well, the mind is nothing but a plaything of the body, correct?"
 * In Gunnerkrigg Court, Reynardine's behavior seems to match his outward form: when he's a pint-sized plushy, he's snarky and generally unpleasant; when he's a Big Badass Wolf, he's helpful and seemingly concerned with Antimony's well-being. It's unclear which is the cause and which is the effect.


 * Word of God says that he changes form based on how he's feeling. That is, Rey was being sarcastic.
 * Wouldn't that make this an example of "The Body Is A Plaything Of The Mind"?
 * Another possible example is Alistair. Word Of Tom says that Alistair now has a bird's brain, and all that entails. On the other hand, even as a bird he seemed to recognize Kat and retain his feelings for her.
 * Later information makes it unclear what, exactly "all that entails": Word of God is that City Face, a short story featuring sapient pigeons, is Canon. So it's not much of a stretch at all to assume that Ali and his parents retained their sapience when they were turned to birds.
 * Further word on the issue: "Sometimes their behaviour can be affected, but the personality is the same. Much like Reynardine."
 * In El Goonish Shive, transformations tend to influence the transformee's behavior somewhat, usually in subtle things like body language (for example, someone turned into a cat-person might find themselves purring without realizing it), and the author has said he enjoys adding goofy little quirks like that: for example he finds it especially funny when Elliot gets genderbent and then acts in a girly way without realizing it.
 * In a bigger way, getting genderbent tends to make one's hormones go haywire. Generally, the transformee gets the orientation that their new gender would normally have in addition to their own (for example, a straight person would turn bisexual, while gay and bisexual people would stay the same), and if two people are attracted to each other normally then they will be equally attracted to each other if both are transformed. The characters eventually get used to it, but it can blindside them if they're not entirely ready for it.
 * The latest comics have featured Elliot forced to go into his "girly forms" with mild personality swaps. Elliot immediately freaks out once he discovers this. Later however, Tedd's Dad reveals that he is able to break character and be aware of the changes. More personality swaps come up later including goth girl and party girl.
 * Ash in Misfile has gone under some relatively minor behavioral changes after being turned into a girl, however, unless s/he's having a period, it's nearly impossible to tell which of the differences are biological and which are due to circumstances. At one time Emily wondered whether differences in male and female brains might be responsible for Ash's poor math skills (it wasn't, as Ash sucked at math both before and after the change). That said, Ash's personality hasn't changed that much, so this is largely subverted.
 * Four of the five main characters of The Dragon Doctors suffer a bit of this as a result of getting gender-swapped.
 * Cole in Char Cole tends to surrender to his Charmander instincts whenever he's in mortal danger, and sometimes needs to be snapped out of it by force.
 * Ozy and Millie presents Pirate Captain Locke, who comes from a world where people are born adults and age backwards. Locke became accordingly less mature as he de-aged.
 * Bec Noir in Homestuck is beholden to Becquerel's canine desires.
 * In Eerie Cuties after a Freaky Friday Flip chocolate craving cute-vampire Nina, and standard vampire Layla find their normal cravings and personalities switching to suit their new bodies.
 * Averted with Ace after a Gender Bender transformation. She just likes to pretend it is this trope, much to the disgust of those around her.

Web Originals

 * In the Whateley Universe, most of the main characters underwent Gender benders when their powers manifested; apparently turning a happy boy into a girl doesn't cause gender identity disorder and since the Unfortunate Implications are glossed over (they get therapy, but it's off screen) to the point you'd rarely realise they hadn't been a girl all their life.
 * Varies from person to person, some change form without any mental effects, some had GID before the change, and its mentioned in passing that some people have developed GID after flipping genders.
 * Main protagonists Team Kimba right now have 7 characters: two had GID before they became mutants; two have spirits (or something more powerful) riding along helping them acclimatize; one seems to just magically be happy going from girl to boy; and two are massively pissed and unhappy about being shifted.
 * It's also been known to affect voluntary shape-shifters, to the point they have classes on maintaining an identity.
 * Its actually specifically mentioned in a few of the original stories (Fey 1 being the one I can find in a pinch) that being an Exemplar (Mutant with Most Common Superpower as an actual power) specifically invokes this trope. That is to say that as a rule, if you are shape-changing due to being an Exemplar, eventually you'll get used to your new body—no matter if it's female, a mutant T-Rex, or a giant piece of sentient coral.
 * The fact that BITs are generally based on the Exemplar's subconscious idea of an perfect body also helps.
 * Present in another capacity with regards to the J-Team, though it's possible that it's just Jade's flair for the dramatic showing through. Whenever she creates another instance of her consciousness to possess something, it acts more or less how you would expect a sentient version of that thing to act. The best example here is probably Shroud/Jinn, who acts a lot more mature than Jade, and pulls off a pretty convincing 17-year-old.
 * Common in Transgender fiction. Victims often have completely new personalities that bear little resemblance to the original man/woman.
 * Equestrian Legends has Meadow. Formerly a pony, now a dragon, and with powerful instincts.

Western Animation

 * In Ben 10 Alien Force, a few of Ben's alien forms, most notably Spidermonkey, cause his personality to change. In the original, the only alien that did this was Greymatter, mostly to fulfill the Intelligent Equals Cultured trope.
 * Ben did say Ghostfreak made him feel weird, for a good reason.
 * Also, happens whenever Ben turns into Rath. He becomes quick to anger in that form.
 * And also his 'Big Chill' alien gave birth to alien babies after it took over Ben's mind several times in order to build a nest. Word of God stated this was due to asexual reproduction.
 * In Beast Wars, a Maximal or Predacon that acquires an organic alt mode also acquires the instincts of the organism they scanned; normally, these instincts are separated fro the rest of their memory by mental firewalls, but various disruptions to those firewalls (from damage to the protoform before scanning or staying in alt mode for too long) can cause the instincts to overpower the Transformer's personality to varying degrees, though a 'bot can delete the firewalls purposely and integrate the two together without any lingering effects. Arguably, a Transformer "possessed" by their beast mode is much more dangerous, as they're forced to obey their unpredictable animal instincts instead of following typical Maximal or Predacon stereotypes (for instance, the Predacon Inferno - who had his personality almost entirely enveloped by his fire ant alt mode - became fanatically loyal to his "queen" Megatron and suicidally and recklessly effective as cannon fodder, all in line with his role as an ant drone).
 * Tigatron however, managed to solve this problem on his own, simply by removing the firewalls and allowing his two halves to mesh. He later passed on the trick to the other Maximals after
 * Subverted and played in a single episode of Extreme Ghostbusters. The Monster of the Week's payload is to occupy human bodies by driving the original mind out. In the case of our Paraplegic Badass Garrett, after his body was occupied by the ghost and his mind driven out, his mind was still drawn in a (wheelchair-)sitting position, while the gang found something was amiss when the corporeal Garrett began walking...
 * Batman the Brave And The Bold: deaging to a toddler doesn't reduce Batman's intelligence, but it does alter his behaviour - forcing him to communicate mainly in baby-talk and draw using crayons. At the end, he's able to figure out a cure for his deaging, but has to tell the Marvel family with incoherent baby noises and pointing at things.

Real Life

 * Lobotomies, anti-depressant and other medications, all sorts of odd quirks and personality changes caused by brain disorders that go away if those disorders are ameliorated, a nice person becoming a cranky one under the influence of stress (or other) hormones, a cheerful Genki Girl who endured tons of Break the Cutie becoming paranoid and bedridden when they get old enough for clinical depression to kick in, Alzheimer's disease: it's very well demonstrated that the personality is a creation of the physical structure of the brain, and even minor physical and chemical changes can turn someone into a very different person, forget turning into a different species. There's also puberty, PMS, or menopause, all of which tend to induce mood swings ranging from inconvenient to debilitating.
 * Philosophy, religion, spirituality, and cultural belief all aside, medical science agrees with this trope. It's why medication has taken the place of psychoanalysis, for example. Unfortunate Implications for transsexuals who believe they're born into the wrong body- that's scientifically impossible, the body you're born with is not only the right one, but the only one.
 * The Proteus Effect - peoples avatars in virtual worlds affect how they interact.