The places we live, work and play in have an effect on us. Usually it's subtle, and in fact it's far likelier that we end up changing the environment to suit our own needs. Not in this place, though. Maybe it's deeply infused with magic, or perhaps the universe itself has a different set of natural laws. Either way, the place changes you, mentally or physically.
Compare Fisher King, in which the land changes to reflect its ruler (and/or vice versa).
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Anime and Manga
Cephiro in Magic Knight Rayearth, which is revealed midway through the first season & becomes a major plot point in the second season.
The Cat Returns: The longer Haru stays in the Cat Kingdom, the more feline her appearance becomes. She changes more if she starts to "lose herself" in the world.
Used in Catnapped, when the kids enter the cat world and turn into cats themselves. It's explained that instead of getting sunburns, the sun in that world turns people into cats. And if you stay there too long, you turn into an Eldritch Abomination, because... um... that's how radiation works.
Princess Tutu is apparently set in real-world Germany, possibly in the modern day, but visitors to Gold Crown Town seamlessly, instantaneously, and invariably become part of the fairy tale, anthropomorphic dancing animals and all.
Being a resident (or even a previous resident) of Hinamizawa means you get a chance of going all crazy.
The plot to Uzumaki. The town Kurôzu-cho is "cursed by the spiral" causing inhabitants to initially go mad, and obsessed with spiral shapes. Towards the end people start transforming into human-sized snails.
When Dilbert was transferred to accounting (workforce made up entirely of trolls), he began to turn into a troll. See here.
Those Annoying Post Brothers have the ability to reality-jump, and either change into a local form, or not, depending on their whim. They can, for example, change into a giant by going to a world of giants and becoming one, then returning to the world they started from but retaining their giant form. They can always return to normal by simply going home.
A strange example is found in Dungeon Keeper Ami. Due to both Ami's research into improved dungeon hearts and the repeated attacks by Dark Gods, namely The Mighty Tyrant and Crowned Death, Ami's dungeon corruption has begun transforming the worn clothing, ornementation, and general appearence of it's denzins in various ways. This has some VERY Unfortunate Implications because in order to fight the magical attacks of Azzaratha and Crowned Death, the corruption has been aspected to themes of 'Fragility' (in opposition to The Mighty Tyrant) and 'Fertility' (in opposition to Crowned Death). This only plays into Ami's reputation for depravity.
This is seen in other Official Fanfiction Universities as well. There's "Dryads" in the Belgariad/Malloreon OFU, a full range of "aliens" in the Star Trek OFU ... basically if there are nonhuman sentients present in the fictional world, and human fanfic writers who think those nonhumans are "cool", expect to see transformed fanfic writers in the OFU.
In The Vampire Diaries story, "Everything Has Changed", fangirls Taylor and Ashley finding themselves in the show gradually forgetting what they know about the show and being from another world. They are unable to leave before it takes full effect.
Disney's Pinocchio has Pleasure Island, called Land of Toys in the original novel, where you never have to work. The longer you stay there, the more you start to turn into a donkey. Though you could say it's a more literal version of making an ass of yourself.
Earth is one of these in My Little Pony Equestria Girls. Ponies who travel to Earth are transformed into humans (complete with appropriate clothing) and a baby dragon who comes through the portal is turned into a talking dog. They transform back when they return home. It was never revealed what would happen if Earth-native humans (or dogs) took the journey to Equestria.
Films — Live-Action
In the TRON universe, you will go from a normal human (User) to a program if you are digitized into a computer. You get to keep your mind, memories, face, and general body shape, but your clothes are substituted with a neat glowing bodysuit and you are subject to most of the "physical laws" of the computer world.
TRON: Legacy changes this, due to the setting being a different Grid built with User integration in mind. Your clothes go with you when you're brought in, and you bleed as a normal human would. Age is kept time-relevant with the real world, even though time in the grid passes much more slowly (Kevin Flynn looks twenty years older, as he should, but is mentally around 1000 years old due to spending all those years on the Grid).
Users are also apparently a lot hardier and stronger than the average program, and Flynn can to some degree alter the computer world around him at will.
The former is probably meant to reflect data size and redundancy. Randomly change a few molecules in a human and a couple dozen instructions in a program's code - most of the time the program will suffer far more for it.
Stay Tuned was about a man and his wife who get Trapped in TV Land via some sort of magic satellite dish given to him by a Corrupt Corporate Executive who was in charge of producing a cable TV network for his boss, Satan, and did so by trapping hapless mortals and torturing them in the shows. They changed to fit whatever "show" they were in (the most prominent example being when they ended up in an ersatzTom and Jerry show as cartoon mice).
Captain Hook invokes this in Hook, claiming that he is Neverland. It seems to not be true, or at least not as true as Hook thought it was, but it's never directly addressed after that.
The monster world in Little Monsters gradually transforms children who visit it into monsters if they spend too much time there.
Milo and the Phantom Tollbooth. As Milo travels through the tollbooth, he changes from live-action to a cartoon (and back again in the opposite direction).
In Pleasantville, the main characters turn greyscale and are wearing '50s attire upon appearing in the universe of Pleasantville the '50s sitcom.
The minor Philip K. Dick novel Eye In the Sky had this to some degree for all the worlds visited. "Justified" because these worlds were all inside people's minds, based on their own strong/crazy worldviews.
In "Honeysuckle Cottage", a short story by P. G. Wodehouse, the protagonist (a mystery writer) gets left the cottage his aunt (a romance writer) lived in for years. While trying to write his latest mystery, characters and events from his aunt's work start showing up in his. He is obliged to flee the place and give up a substantial inheritance.
In the story Dark They Were, and Golden-Eyed by Ray Bradbury human colonists on Mars (it's perfectly habitable there) are slowly converted into Martians. External changes are subtle (see the title) but the personalities are completely rewritten and they completely forget their human pasts.
In H.P. Lovecraft's The Colour Out of Space, a meteorite containing a substance of a colour and an element unknown to man lands by the well of a farmhouse and is analysed shortly before it simply disappears. The laboratory's sample, too, vanishes. Then, later... all about the well the grass wilts, and dies. All about it the plants wither, and crumble to dust... but not before deforming, mutating, and taking on the colour. The farm animals, too, become sickly and die and even the family living there is affected as they start seeing things and slowly descend into madness as they waste away... and at night they swear they can see flashes of something, the colour, moving in and about the well like mist, or fog... and then, when they are all dead and/or gone, almost all the colour draws itself out of the farm and its environs and flashes skywards in a series of pulses... some comfort considering that the entire valley, well included, is about to be flooded to make a drinking-water reservoir. Whether the colour is sentient or even alive at all is never established.
There's a canon example in Quag Keep and the sequel Return to Quag Keep by Andre Norton- a group of humans are brought in to the world of Dungeons & Dragons, but in the form of their player characters. As a result, one becomes an elf and another a lizardman.
An Elegy for the Still-living: The entire story takes place in one of these, though it is most obvious in the second chapter. Francis creates a world from his own beliefs, and it in turn changes what he believes.
Played with in Terry Pratchett's Wyrd Sisters. Given the old age of the kingdom of Lancre, Granny Weatherwax states that the land itself has come to developed a sort of mind, which, while it cannot change the mood of the people, can sense the people's mood (Especially its king's), and if they don't love and appreciate the kingdom, It might get angry. Which would change the people's mood when you consider multiple tons of tectonic plates throwing a tantrum.
In The Cineverse Cycle by Craig Shaw Gardner, characters' personalities shift depending on what movie world they're in. Usually this is subtle, but each character has one world where they will shift drastically to fit; most notably a Mad Scientist who warps into a hapless hunter when he's confronted with cartoon bunnies.
In Alice in Wonderland has been hinted at by the Cheshire Cat that staying in wonderland too long will drive anyone mad, or possibly more accurately, you can only enter if you're already mad to begin with.
In the Graphic Novel "Amulet" there is one town, that is cursed. The longer people stay there they gradually transform into anthropomorphic animals. Though Leon was turned into a fox, as was his father. So it seems the spell has some bizarre connection to genetics.
The Doctor Who serial Survival has the planet of the Cheetah people, which gradually turns you into a Were Cat the longer you stay.
In Asylum Of The Daleks, there is a Dalek Prison planet orbiting Skaro, which has some sort of nanobot-virus on the surface, that anyone who comes in contact will be transformed into a Dalek.
The Space Cases episode "Tie Me Kangaroo Down, Court" where the ship ends up in a parallel universe and everyone starts turning into Spung, the villainous lizard people of the series.
In Babylon 5, it is said a few times that no one leaves Z'ha'dum the same person as when they came.
Mythology and Religion
This concept is central in Feng Shui (and geomancy in general). And you can alter your environment to make life better for you.
The TORG game had several parallel Earths invading "the real world", and each one had a tendency to warp the new inhabitants to the new rules of that domain. Player characters had the ability to resist this effect to some degree.
The old AD&D cosmology (previous to 4th Edition) featured the Outer Planes of Elysium (pure Good) and the Gray Waste of Hades (pure Evil). A non-outsider on Elysium experiences increasing joy and satisfaction while there and finally has to make a will saving throw or fall under the control of the plane, becoming a petitioner of Elysium. In Hades, a non-outsider experiences increasing apathy and despair : colors become grayer and less vivid, sounds duller, and the risk of entrapping is the same as in Elysium (albeit less nice).
Not to mention how certain layers of the Abyss will change you into a bodak just for visiting them.
Other settings that don't use the standard cosmology may also include planes or regions with the "entrapping" trait of Elysium and Hades (like Dolurrh in Eberron).
This is how Arcadia works in Changeling: The Lost. Abducted humans find themselves forced into a role by the True Fae; similarly, the laws of physics in Arcadia have been thrown aside in favor of contract law, so they need to sign onto their masters' Contracts in order to survive. Both combine to physically twist the human into a new role — be it a loyal hound, a perfect lover, or a tree.
In Ravenloft, anyone from outside Darkon who stays for too long will have their memory altered to believe that they had always lived there, even to "adopting" random tombs as belonging to ancestors. Natives euphemistically refer to the phenomenon as "Finding one's roots". The Necropolis is more blunt: enter, and you die and become one of the Undead residents.
From "Expedition to Castle Ravenloft", Strad has a connection to Barovia... literally. He gains supernatural powers on top of being a super-vampire from three fanes that makes him virtually impossible to kill.
A more nightmarish variation comes from Warhammer 40,000 (who else?) in the form of the Eye of Terror and certain Daemon-infested planets within, in particular the Planet of Sorcerers. The longer you stay on one of these Daemon worlds, the more mutated you become, until you become a mewling, degenerate lump of writhing flesh. Ahriman of the Thousand Sons attempted to use his magic and try and save his troops from mutation. It, um, didn't work out too well.
Evidence also suggests that the Eye of Terror may be similar to the The Cat Returns example above (just not involving cute Cat Girls). In the 4th edition Chaos codex, it mentions that the only thing keeping the Chaos Marines alive, (mostly) unchanged, and (relatively) sane is their sense of purpose and undying hatred for all things Imperial.
The magical forest of Athel Loren from Warhammer is having this effect on the Wood Elves. As time goes by, they become and act more and more like trees (aggressive in summer, passive and torpid in winter) and develop a deeper and deeper connection to the forest spirits, although unlike most other examples here it's an incredibly slow (generations-long) process.
This happens in Dont Rest Your Head to most of those who get lost in the Mad City. They slowly lose their real selves until all that's left is their profession. They will just start to work ceaselessly until something happens to kill them. The only people immune to this are the Awake, and even they occasionally submit to it willingly given the alternatives
The Wyld in Exalted, home of The Fair Folk, has a stronger and stronger mutating effect on those who enter it the deeper in they travel. The Exalted are much more resistant to it than mere mortals are, however—and thanks to their tattoos, the Lunar Exalted are almost entirely immune.
Yu-Gi-Oh! has Zombie World, a field card that turns all monsters on the field and graveyards into zombies
Silent Hill and its inhabitants change to reflect the inner turmoil of the protagonist. Which would make it the Life Link kind, with hints of Physical Warping: the protagonists get randomly teleported into different dimensions or locations and may have their clothes changed. The protagonist Murphy Pendleton of Silent Hill: Downpour gets turned into a Humanoid Abomination monster, and back again, during the course of the story.
In The Clue Finders 6th Grade Adventures: The Empire of the Plant People, one of the team members is captured and starts to turn into a sentient plant herself. This is stated to be the effect of her drinking the polluted water the plant-people are forced to drink; the game gets heavily into its Green Aesop near the end. Whether that implies that the plant people were all or even partially originally human is never adequately explored.
The Dark World in The Legend of Zelda: A Link to the Past turns anyone who enters it (without the Moon Pearl, that is) into a form that reflects their soul: Two notable examples are Ganon, who is turned into a boar, and Link, who is turned into a little pink bunny who can't use any of his items except the Magic Mirror (which allows him to return to the Light World).
The Twilight that covers Hyrule in The Legend of Zelda: Twilight Princess functions similarly (creating a slightly different and more evil version of the Light World) but rather than reflecting their personalities, it causes the people of Hyrule to become spirits that are unable to interact with Link. Link, however, turns into a wolf, because he is the hero foretold by prophecy to save Hyrule and become a "blue-eyed beast," rather than a spirit, when exposed to the Twilight.
In Mass Effect, the Reapers will gradually Mind Rape anyone who spends too long inside of one into a loyal worshipper. The second game reveals that a Reaper corpse that has been dead for over 37 million years is still capable of indoctrinating people!
In the Kingdom Hearts series, Sora, Donald, and Goofy (and, later on, Riku) may magically transform into bodies more "appropriate" to the world they are visiting. This is relative, however, because apparently it is more "weird" for an upright-walking talking duck to exist in the Pride Lands than it is in Port Royal. It seems to be dictated by the Rule of Funny and Rule of Cute than anything else. Justified in Space Paranoids and The Grid, however, because it's well known from the films that digitizing people makes them neat and glowy.
Technically averted in most cases, as the characters are transforming themselves to blend in. First through Donald's magic, and later with Sora's new clothes. Timeless River seems to play it straight for the heroes, though for some reason modern-day Pete is unaffected.
In Final Fantasy Tactics Advance The real world is transformed into a copy of Ivalice, unlike the other games in the Ivalice series which play it straight as a separate world entirely. Everyone is transformed into the various denizens, either human or becoming another race entirely, or even Monsters. Their memories and personalities are rewritten. Except for the four main characters who made the wish. Even Cid despite he eventually fully believes what is going on, still doesn't remember his other life.
Hell in Judecca works this way. The longer you stay the more likely you are to transform into a ironic caricature of who you were when you were alive.
Parson Gotti of Erfworld finds himself subject to the first variant. For example, any profanity he tries to utter comes out as "boop". Later it turns out the spell that summoned him included a subtle Mind Control aspect that made him more ruthless.
Danielle Kendall of Kevin & Kell is turned from a human to an anthropomorphic rabbit when she crosses over into the dimension that consists only of anthropomorphic animals.
It happened in reverse to Ralph and Martha much earlier in the strip. Ms. Aura and her child went from Vulture to Dolphin and eventually to human.
The Sanrio universe is this to Lovecraft's Elder Gods in Hello Cthulhu.
The worlds of City of Reality do this courtesy of their different Auras, which define each world's physics. The Aura Stone of a given world carries that world's Aura within it, and can affect someone from a different world regardless of location. For example, an element exclusive to Reality turns to water and poor Hawk (an insect "dragon") turns into a blind human when exposed to Magic World's Aura.
Due to its Genre Roulette nature, the Book in The Book of Stories OCT can change the setting and archetypes inside on a whim. It's implied in some entries that the Book might start to affect some of the contestants as well in this manner.
A weird variant in Sponge Bob Square Pants: everything underwater is rendered in cartoon form, while (most) everything on the surface is done with live action. On one occasion, SpongeBob and co. venture ashore, only to turn into puppets (a sponge on a Popsicle stick, anyone?) while on land.
The Mind Screw film The Elm-Chanted Forest has the main character fall down a hole and get captured by humanoid mushrooms. They tie him up so that eventually he will turn into a mushroom as well, because "Everybody becomes a mushroom down here". Except for a talking snail, apparently because the mushrooms have never noticed him. And then they sing a psychedelic musical number, starring a fungal Expy of Michael Jackson and Prince. And some of them appear to be wearing blackface. Did I mention this movie is a Mind Screw?