Mariana: Don't pee in the water. Beck: Why? Mariana: A candiru, a vicious parasite will swim up the urine into your pau. Beck: Swim up my what? Mariana: Your pinto. It'll swim up your ding-dong. And once it gets in, you can't get it out. Beck:(stammers) Well, then what? Mariana: They have to amputate. Beck:(closing his pants really tight) Not this boy's pinto. Uh-uh. Not today!
In Dragon Ball Z, one of the first acts Super Buu commits after being formed is to turn entirely into pink goo, force himself down the throat of one of the humans that killed Fat Buu's dog and expand, blowing the guy apart from the inside.
Later tried again against Vegeto, who responded by beating the crap out of Buu from the outside.
This is how the larval form of the entities in Parasyte take control of their hosts, usually entering through the ear of a sleeping person to get at the brain. When the protagonist fell asleep with headphones on, his symbiote tried getting in through one of his nostrils instead. After he woke up, it had to settle for burrowing into his arm. He tied his headphone wires tight around his arm to keep it from burrowing up to his head before it went in place, screwing up the transformation process so he and it are Sharing a Body instead of it taking over completely.
Getter Robo has the Invaders, a race of cosmic horrors who can invade people's bodies if they so much as bleed on them. Though most of the time they prefer to just dive into people's throats, or simply rip a hole in their torso and use that.
Medusa in Soul Eater possesses an innocent girl in the suburbs this way, by turning into a snake and slithering into her mouth.
And before she did it to the girl, she did it to a dog.
She also does this with the snakes she controls instead of herself: if she can get something into an entry point to your body she can send in hundreds of snakes which let her track you wherever you are and can rip you to pieces whenever she tells them to.
In Basilisk, one of the ninjas from the Iga clan whose power is to turn into a slug-like thing in the contact with salt kill a ninja from the Kouga clan by jumping in his mouth, inside his throat and breaking his neck.
Done constantly to poor Sakura in Fate/Zero (with a pit full of worms) by her stepfather.
The DC Universe52 series answers a question no one ever asked before: can Orifice Invasion of a robot be made to look horrific? The answer? Dear God, yes.
In Calvin and Hobbes, one of Calvin's fantasy sequences involves a frog forcing its way into his mouth and being swallowed. Which leads to the punchline: he had a frog in his throat.
The image example for this trope comes from The Piper #2 by Zenescope comics. The woman in question (Sandra) just had a poisonous snake slide into her mouth after putting her lips on a french horn's mouthpiece, all thanks to the Piper playing his lethal song outside her room. She dies.
The Hidden: The alien parasite burrows into and out of mouths several times during the movie. One scene used a prosthetic head to show the process in gory detail; upon seeing that scene on film, the actor whose likeness was on the head was physically ill.
A "bug" in The Matrix enters through the bellybutton.
In the short film Ghosts, the Maestro (played by Michael Jackson) possesses the Mayor (also played by Jackson) through the mouth.
In The Island, the diagnostic sensors (which look like a cross between ticks and spiders) crawl into the body via the eyesocket.
In John Carpenter's Prince of Darkness, the Anti Christ is a green liquid who enters into people via their mouths: the hosts then do the same thing to spread the possession, i.e. vomiting into other people's mouths.
The titular Cat from Hell (a Stephen King short story adapted into the anthology Tales from the Darkside: The Movie) enters through the mouth of a hitman hired to kill him, causing that man to choke to death before the cat goes in all the way (we read/see just the tail wagging out of the mouth before it disappears into the victim). This really goes with Rule of Scary in The Movie, where the cat is obviously way too big to do that (although vaguely hinted to be a spirit of vengeance).
In Animorphs, Yeerks take over human beings by way of infestation through the ear canal.
Also Father in the Ellimist Chronicles, a giant sentient sponge (well, sort of) that sticks tendrils into the body to keep its victims alive (or in the case of the dead, keep them from decaying as long as they remain attached) and giving it access to their minds.
In the short story "Motherhood Redeems Women" by D. Douglas Graham, an aborted fetus decides to return to the womb by the same door he came out. Squick ensues.
There's a story by Tanith Lee about a demon that takes possession of humans via their orifices, so to try and prevent being possessed one guy blocks all of his orifices, but forgets his urethra. Ow.
In the Star Trek: Deep Space Nine novel "The Siege", an evil shapeshifter kills a Cardassian by ramming himself down the victim's mouth, then expanding inside. In a later novel, Odo threatens to do this to a Cardassian officer should he dare to disturb Odo's regeneration.
The biotech of the villainous Yuuzhan Vong in the Star WarsExpanded Universe often does this. Numerous creatures fit the trope more exactly by slithering into ears or eye sockets, or down the mouth.
Spore, in Galaxy of Fear, combines this with evacuations. Its hosts shoot vinelike tentacles out of their mouths and eyes; these sink into the skins of new victims and they are converted into new hosts within seconds. It doesn't actually leave its current hosts - its unwillingness to ever let go of anyone it's claimed is marked.
In Luke Skywalker and the Shadows of Mindor, meltmassif/the Melters enter as spines thinner than needles through people's pores. When they're purged, they ooze out of pores and eyes and mouths as oily liquid.
In Stephen Baxter's Space, the hero Malenfant is integrated with an android/computer system. Tendril-probes infiltrate him... essentially everywhere, and there is no attempt at anaesthesia. He is doomed to live for billions of years in this state unless he fails in the task for which he was converted, in which case a stellar event will occur that will kill every living thing in the galaxy except for archaeobacteria and slime moulds.
The Fallen. Leviathan'' has people having their bodies taken over by making the creatures enter through their mouths.
It's one of the trademarks of The X-Files. The Black Oil a.k.a. Black Cancer a.k.a. Purity is an alien virus that gets into your body through your eyes and mouth and assumes complete control over it, optionally using it as a host for gestating a baby alien.
One Real Life example was in The Amazing Race, where one of the guy woke up partly covered by leeches, and one somehow crawled into his urethra.
The prehistoric worm parasites in the revived The Outer Limits episode "From Within" entered (and later exited as they died) through nostrils, mouths and ears. One girl actually had a worm go in her right ear (complete with blood) and at the end of the episode have it come out her left ear without leaving her with any ill effects (other than a great deal of pain).
Doctor Who TV Movie: the master in the form of a slug like creature crawls into the mouth of a sleeping paramedic to take control of his body.
An episode of Lexx had space carrots that take over people's bodies by ramming themselves up their anuses.
In a bit of a twist, through the mouth is the more pleasant way for a Goa'uld symbiote to enter a human host: Goa'uld normally enter through the neck, not wishing to see the expression on their future host's face. Only the Tok'ra, a breakaway group who only accept voluntary hosts, normally enter through the mouth.
Non-Tok'ra will enter through the mouth to avoid leaving a visible scar, however — if they have reason to suspect they will be attacked if discovered.
The creature that inhabits Helen Magnus of Sanctuary has a parasite exit through her ear canal after she dies. Based on the dialogue, it got in through her pores.
A classic Night Gallery episode has a man hiring someone to do away with a rival by planting an earwig in his ear at night, where it will crawl into his head, constantly eating (see "Real Life" below) — in a karmic slip-up he gets it planted in himself. In a million-to-one fluke, he survives weeks of agony when it crawls out his other ear. With the boldness of one who's been through Hell he owns up to his deed and claims he'd do it again...then he finds out the earwig was an egg-laying female.
Hunting for mole lizards in Baja California, the host of Weird Creatures is told a local Urban Legend that these worm-like animals will invade the anus of anyone who defecates over their burrows.
In Series 4 Episode 4 of Misfits, Rudy sleeps in a slug-infested room in the community center, and one of the slugs crawls up his anus. He attempts to remove it by inserting some salt.
In Supernatural, demons usually enter their meat suits' mouths as smoke.
This is how the Nogisune tends to possess or influence people in Teen Wolf. It tok control of its first body by entering it's mouth the form of a fly. Later, while in Stiles's body, it released many more flies. They entered Isaac through his IV pump, Ethan through his nose, Aiden through his ear, and Derek through an open wound on his back, and acted as a Hate Plague for all four of them.
D&D 3.5 edition sourcebook Lords of Madness introduces a new (and decidedly creepy) aberration: the Tsochari (singular Tsochar), a mass of tentacles that bores into victims and can either coerce the victim into behaving itself by causing them great pain, or, kill the victim and wear the body as a disguise.
And then there's the process of ceremorphosis, which a mind-flayer tadpole is inserted into the ear of a hapless humanoid (usually a captive human, orc or drow), which eventually turns them into a new mind flayer.
And to crown the collection there's the DC 90 Escape Artist check from the Epic rules. Combined with Enlarge Person...
Combined with the Exemplar class, who can among other things do stuff like substitute a Diplomacy skill check with Escape Artist, you can even do this and impress onlookers so much they will adore and admire you.
The alien species in Microsoft's Freelancer entered a host through the mouth and effectively take complete control of the body, including the host's memories and personality traits, or at least enough so that nobody realizes that the person they're talking to isn't really the Chancellor anymore...
In one of the dungeons in Pokémon Mystery Dungeon: Explorers of Time/Darkness/Sky, Grovyle is attacked by a Spiritomb, which took control of his body by entering through his nose.
This is how the demonic spider Lucifero entered Yuko Amano's body in Fight or Die. through her navel
Yeon from Tower of God falls victim to a Puppeteer Parasite that invades her gastro-intestinal tract. The fact that it was as big as her and disguised as a Monster Clown makes it just as squicky as the fact that the way how it entered her mouth on the bed was shot made it reminiscent of something else entirely.
The...uh...nurses(?) in Oglaf use a "polyp of cleansing" to cure the unfortunate Ivan of poison. The phrase "open your food tract, mammal" is employed, followed by a very unpleasant-looking panel. Of course, given which webcomic this is, perhaps we should be glad it was his mouth.
In the animated short "The Pier", a strange creature stalks a bird-man as he fishes for a meal. The creature regurgitates one of its fish-like young into the water, and the child is "caught" by the bird-man who promptly swallows it whole. The baby then starts nibbling on the bird-man's stomach lining, causing him to collapse in agony. The creature grabs the bird-man and forces even more of its young into his throat. The short ends with the children gleefully munching away. Moral of the story? Always chew your food.
In an episode of The Venture Bros., The Monarch threatens to dunk the Venture family in a river teeming with candirus. Bizarrely, Dr. Venture claims the candiru are a myth (making it unclear if it's a mistake by the authors of the show or Dr. Venture himself).
This is likely a call back to the first episode where Dr. Venture insists that Chupacabra are not real but later finds dozens have stowed away on his jet. Also, though Candiru are real, the medical community has pretty much cried foul on its alleged method of attack, so Dr. Venture might simply have meant the whole "swimming into your urethra" is a myth.
There's an episode of Batman Beyond where the villain, an amorphous woman named Inque, tries to suffocate Terry by forcing herself down his throat.
In Code Lyoko, XANA's specters generally possess people by entering through the mouth or the ears.
In episode "Franz Hopper", instead of possessing one specter tries to clog Jérémie's airways by entering his mouth and nose.
Legion Of Superheroes has Timber Wolf's father infecting him with nanites that enter through his ear while he slept.
In one episode of The Simpsons, Groundskeeper Willie fills the auditorium with rats in revenge for being humiliated. Bart warns Milhouse not to open his mouth; of course, Milhouse starts to say "What?", at which point half a dozen rats leap in.
Happens to Sokka when Momo grabs a spider out of his mouth to eat it.
Momo is missing, and Sokka thinks Appa ate him, so he crawls in Appa's mouth to see. Appa just spits him out.
Done in Sylvester's first short Life With Feathers, in which a henpecked male bird wants to commit suicide, so decides to feed himself to the cat. Of course, Sylvester is suspicious about why the bird would want to do this, so he spends the entire cartoon running away, with the aforementioned bird chasing him and pleading to be eaten. Found here.
Sym-Bionic Titan takes this to a disturbing extreme. Xishi, a squid-like monster about the size of an average human, climbs all the way down her victim's throat and forces them to speak the truth. What's really creepy about it is that Xishi's face is visible from inside the victim's throat.
A subversion occurs in Season 4, when Obi-Wan voluntarily swallows a "vocal emulator", a spider-like device that changes his voice, in order to perfect his disguise as Bounty Hunter Rako Hardeen.
Mr. Slave from South Park enjoys forcing small animals to crawl up his ass.
The candiru. Unlike this fish, however, more parasites make a habit of leaving through various orifices, but they usually enter as eggs and grow inside the body.
Generalizations about the candiru's hunting methods rest on very little evidence, as there has been precisely one documented case of a human attack, in 1997. In particular, it is not chemically sensitive to either ammonia (excreted by fish) or urea (by humans).
There's also an urban legend about earwigs crawling into your ears (hence the name), although they don't actually do that.
Earwigs, as well as spiders and other creepy crawlies, actually do prefer small, tube-like spaces, and will occasionally find their way into someone's ear canal. They'll usually just leave as soon as they discover it's coated with slippery and unpalatable earwax.
Some Argentinian torture methods during the Dirty War involved a bunch of naked people in a small room, in fetal position, forming a circle. Then, their torturers dropped a starving rat inside the circle the people were making. The rat would try to escape by entering someone's ass, literally.
This is how most scavengers consume carrion, as it requires the least amount of effort to get to the soft fleshy bits.
Ants, such as army ants, will often attack a larger creature's soft spots in order to bring them down or drive them off. People being attacked by army ants can expect—in addition to painful bites and blisters—to get ants in their mouth, nose, eyes and ears. There have been a few cases where ants have killed infants and small children by invading their mouths and noses and more or less causing them to drown in biting, stinging ants.