Done in one of the Van Beuren Studios Tom and Jerry shorts (no, not THAT Tom and Jerry)—and Played for Laughs. In one episode taking place in the Swiss Alps, the duo eat a strange type of cheese that causes swiss-cheese esque holes to pop open in their body.
And before that, he'd been suffocating Batman with an attack that was lifted frame-for-frame from Akira. TMS Entertainment worked both on Akira and this episode.
Poison Ivy, who made a habit of creating plant creatures that could pass for human. These often revealed themselves to be horrifying monsters. The best example of a horrific transformation is where Ivy rips the skin off of the upper torso of one of these creatures like it was a shirt, revealing the green flesh underneath. This is enough to cause onlooker Robin to start to heave (he's saved from actual puking by Batgirl).
"House and Garden", where Poison Ivy has appeared to reform and married a college professor and his two sons... only, it turns out the professor and his children are clones made from plant matter. And they only stay "human" for three days before turning into something that looks like Swamp Thing after a lawnmower accident.
At the end of "Bane," the titular muscleman gets a little too much Venom. His appearance in Batman & Robin absolutely pales in comparison.
Two-Face isn't just scarred on the left side of his head, but also has scarring on at least his arm.
Inque, a shape-shifting mutant who provides plenty of canonfetishes for the technician who releases her from a cryogenics facility. She dupes him in the nastiest way imaginable. She also likes to engage in Orifice Invasion and at one point tries to suffocate the protagonist by forcing herself down his throat.
In one episode, an android contorts his limbs horribly in order to chase Batman down an elevator shaft. The commentary mentions that this was an homage to Legend of the Overfiend.
"Splicers" is about this new fad of people (mostly teenagers) getting some animal genetics mixed with their body so they look like human/animal hybrids, including a bull, a snake, and a big cat (sounds really familiar for some reason) . Even Terry McGinnis involuntarily becomes an actual batman until he got better. However, the really bad part is when, after splicing becomes illegal since it makes people more aggressive, Batman goes to confront the guy who invented the process, who then turns himself into some weird Hawk-tiger-snake thing he calls a "true Chimera" and after knocking away Terry's gun filled with mutagen cure-all, Terry starts sticking the villain with multiple syringes. The villain then transforms into a disgusting creature that is even today one of the most disturbing things ever put into a kid's show. Unless you want to possibly be scared witless and lose some sleep, Take Our Word for It. Otherwise...◊
Cuvier: WHAT HAVE YOU DONE TO ME!?
Jackson Chappell, who is rendered an obscenely muscled but brain-dead vegetable after an overdose of steroid patches. He didn't look so good before that, though; like his charge and employer Bane, the overdose seemed like it was about to tear his muscles through his skin. And speaking of Bane, he wasn't doing too well in the episode either, going in the opposite direction entirely: He seemed to have lost the entirety of his musculature to venom abuse and age, leaving him a bag of squalid, semi-catatonic skin and bones.
The dog fighting club whose owner is using steroids on the dogs. The first test subject is... a bit overdone.
"Earth Mover" featured a rotting, living corpse, who was buried alive with a radioactive chemical that fused his body into the surrounding dirt. After years of being trapped underground, he looks like this. He's still alive.
There's also Magma, one of the Terrific Trio from "Heros". Imagine Clayface made of magma and you go the idea.
Big Time from "Big Time" and "Betrayal" was at first a big-mouth teen before mutating into a grey, hulking brute.
Terry's arch nemesis from the first season, Blight. After getting radiation treatment from his own nerve gas, he turned into a living blight emitting radiation that ruined anything (or anyone) he touched. He looks like a blackened skeleton surrounded by a green glow in the shape of a man. He had fake skin that eroded away when he got angry and let out excess radiation. Over time, the radiation ate away and his sanity and caused him to eventually lose his mind and fully assume the role of Blight.
In Justice LeagueBrainiac morphs Lex Luthor's body into some bizarre hybrid of biology and technology. Not to mention revealing itself involved tearing its way out of his body, somehow bloodlessly but still ripping holes in his torso and entirely destroying his limbs.
There's also an earlier scene in the same episode where Supergirl kills Galatea by... smashing a giant freaking power cable from the Watchtower's main reactor into her stomach, right as it's being turned on. As Supergirl and an injured Steel limp away, you see Galatea twitching, her skin grey and a massive scar on her stomach. It's not entirely certain that killed her, but that might only make things worse.
Some of Ben 10's alien transformations involve this. He grows an extra set of arms when turning into Fourarms, eyes pop out of his temple when he turns into Stinkfly, and so on. This is probably an Affectionate Parody of the scary alien transformation scenes so typical of horror films.
In the first series, the Stock Footage transition montage could become this as well.
The mutations inflicted by Corrodium in the Ben 10 episode "Under Wraps" were particularly unnerving; monstrous, twisted, black shadows with glowing purple lines gouged across them. Later on, in "Be Afraid of the Dark", when we actually see the mutations taking place as the entire populace of Earth is subjected to them, it's even more disturbing.
Perhaps this is why the Alien Force transformations are more boring less graphic...
The final part of Ben's initial transformation into Benwolf, in "BenWolf". Sure, it only lasted for a few seconds, but...
Invader Zim. In one episode he gets a huge pimple that appears to hypnotize everyone around him and then it explodes and floods the entire school. In another one, he collects organs from children and stuffs himself with them, replacing the organs with various, painful objects that he inserts in those kids' bodies. There is another episode where both Dib and Zim slowly transform into bologna. And then there is the episode where Dib has a huge hole in his head that absorbs nightmarish creatures, along with Zim. And, well, it's Jhonen's show after all, so every episode has its fair share of horror.
A special mention for that one time Zim gave Keef a present that ripped his eyeballs out and replaced them with mechanical ones.
The Parking Garage Rat People in Game Slave 2 and when Zim's eyes BUBBLE and BURN in Walk Of Doom.
Catscratch has dealt with this in a few episodes- one being Major Pepperidge where Mr. Blik grows a second head (one that looks like him, but talks like a Jazz musician) and "Lovesick", where Gordon has an allergic reaction to broccoli where he puffs up really bad, gets a huge blood shot eye, a rash all over his body, razor sharp teeth and a drooling tongue in his mouth and a huge arm with razor sharp claws on his paws... This happens again at the end after Kimberly gives him chocolates which he also is allergic to.
Courage the Cowardly Dog has tons of these, being a horror show and all. Most of them are shown when Courage does his signature screams.
An episode of I Am Weasel had Weasel as a doctor who specializes in curing diseases; we see him with a guy who's got a case of Athlete's face (a foot sticking out of his forehead). I.R Baboon is stuck having to study parameciums (amoebas). Weasel takes a break for a while and I.R decides it is his time to shine - so he invents a so called "cure" for Athlete's face and uses it on the guy - it turns the guy into an amoeba-like form, he uses it to "cure" everyone else's problems and they get turned into amoeba-like things too. Weasel makes the real cure for the first time and tests it out on the Athlete's face guy but the athlete's face guy turns into something with only a head and a foot, so he tries again and it's successful. Thankfully, he shows up with the real cure and everyone returns to normal at the end.
One episode of Danny Phantom has the main character trying to duplicate himself, only to horribly, horribly mutate himself: Little heads on individual fingers, mouths for eyes, eyeballs sticking out and so forth. On the other hand, it was Played for Laughs.
Dexter's Laboratory has a direct American Werewolf parody called "The Laughing" (a reference to the werewolf movie The Howling) where Dexter becomes a "were-clown" after being bitten by a pair of dentures belonging to a clown performing at Deedee's birthday.
There's also the horrifying transformations in "Monstory".
Parodied in an episode of Futurama titled "The Honking", which had Bender turning into a psycho werecar in one scene. But after a while he kind of gets used to it and it begins to actually feel quite nice.
Although it's partly Played for Laughs, the hip 80's CEO dying from boneitis in "Futurestock", complete with delightful bone-snapping sounds as his body constantly distorts into unnatural shapes. It doesn't help that when he dies, both of his hands pull his eyelids and lips back, freezing his face that way.
Gargoyles, "Metamorphosis". Elisa's younger brother gets transformed into a gargoyle-ish monster, and the only scientist who understood the process is killed. The episode ends with our cool veteran cop, Elisa, crying uncontrollably at the fate of her brother.
The Grim Adventures of Billy & Mandy has had more than a few episodes in which this has happened. One episode has Billy suddenly gets a pimple on his back which turns into a creature that acts like him who is dubbed Yupp Yupp. Billy is a king at this trope. Like the time he got his face ripped off. And in the intros, when the characters are brought from skeleton to muscle to skin.
Then there's the episode with the zipper, and he uses the zipper to remove his left arm and replace it with the cat's head, and in "Prank Call of Cthulu" when Billy's nose changes into a squid-like appendage which can flip up and spawn hundreds of insect-like creatures with Billy's head, sharp teeth and tiny tentacles. Screaming wildly, they all fly to Grim and attack him.
One episode had Mandy slowly transform into a Giant Spider, much to Billy's arachnophobic horror. For Billy's "son" Jeff, though, it's love at first sight.
Another Billy example: Billy is transformed into a chocolate sailor, and he's just so delicious that he can't stop eating himself. He screams with every bite, until he's just a head.
A few episodes of Mr. Meaty have dealt with this. Often it's due to the poor quality of the fast-food restaurant's entrees. In one episode, Parker mooches off most of the other characters and then gets infected with a tapeworm, "the ultimate moocher", which is later eaten by a scientist who collects internal parasites.
In the episode "Parkerina", Parker eats too much Ms Meaty burgers and undergoes an American Werewolf In London style transformation into a girl.
An episode of The Real Ghostbusters had Egon turn into a werechicken, while another episode had the whole team undergoing a number of bizarre and sudden transformations after becoming "allergic to ghosts".
An episode of Extreme Ghostbusters had one of the lead characters becoming an evil clown-like creature. Then there's the episode "Crawlers" where people's lungs were used to host the Monster of the Week's larvae, and Jeanine was turned into the monster's mutant bug creature-bride; "The Ghostmakers," where Eduardo's spirit is forcibly displaced from his body by a demon with a magic mirror; "Slimer's Sacrifice," where Egon is transformed into a pterodactyl-like harbinger of Ragnarok; "Be Careful What You Wish For," where a Literal Genie graphically transforms his victims into a tree that takes root, or a pile of money that blows away in the wind; "Eyes of the Dragon", where bones are stolen from living victims...really, someone on the production staff had to have a body horror fetish.
The Vathek, a trio of "literary" ghosts from the episode Deadliners. Visually, they're expies of Clive Barker's Cenobites who perform on unwilling victims "unnecessary surgery" that would make Ivo Shandor put away his tools in disgust. The worst part? The Ghostbusters are unable to put a dent into them because "if it is not written, it cannot be done," given as how the writer who brought them into this world never wrote that they were vulnerable to proton beams. In the end, they were defeated when Eduardo incinerated the typewriter they originally entered through.
In ReBoot, anyone caught in a lost game is transformed into a slug-like "Null".
What's worse is that it's implied Nulls are still somewhat aware, and apparently some residents use them to make balloon animals - complete with painful Null noises.
There's also what happens to people that get trapped in the Web without proper protection. Bob was only in there for a short time and his uniform was warped, his silver hair melted and turned black, his hands were charred black, and he had various scars on his neck and face. And he had partial protection from the Web too. Without that Bob would have ended up like Web Riders, which were in even worse shape.
The Secret Saturdays episode "Ghost in the Machine" features a villain that creates a DNA reactor that merges living tissue. He places Zak, Fiskerton and Komodo into the machine, and... it isn't pretty.
Don't worry. That was just a dream. Everything is perfectly fine. Well, except for that fog that turns people inside out.
In "The Bob Next Door", during one of his escape attempts, Bob surgically removes the face of one of his cell mates and then switches it with his own, while still being fully conscious and screaming. The worst part is when Sideshow Bob stops at a rest stop and a waitress who's takes an interest in him, manages to pull off a loose thread used to keep his face on, causing his entire face to flap down, exposing all of his muscles and tissue. And then moments later, it happens again with his cell mate he switched faces with. Just as horrifying as the movie this episode was a parody of...
Bart imagining Homer's face melting in "Brother from the Same Planet."
The "Trapper Keeper" episode of South Park pays homage to the AKIRA transformation scene (complete with soundtrack); it involves Cartman's Trapper Keeper absorbing all of his school supplies, then his computer, then Cartman himself, turning him into a hideously gigantic bio-mechanical blob monster that threatens to destroy the world via the absorption of NORAD.
Kyle's dad surgically altering himself into a human dolphin. And then there's a lot of the(realistically designed) title characters from Freak Strike.
Poor Spider-Man underwent quite an unpleasant transformation into ManSpider in the "Neogenic Nightmare" arc. Later on, the Vulture absorbed Spidey's youth and powers... and also his unstable mutating DNA, meaning he sporadically changed fully or partly into the same creature.
To avoid the "violence" of sinking his fangs into peoples' necks like regular vampires do, the creators gave Morbius sucking lamprey-like mouths in the palm of each hand. His typical feeding habit was to slap his hands over someone's face and suck out the plasma through them. After getting hit with the neogenic recombinator for the second time he turns into a hulking bat thing.
The Spectacular Spider-Man has Connors' transformation into The Lizard, especially the part where instead of compressing slowly, his head partially implodes to form his new lizard head.
In an episode of Static Shock ("Junior"), the attention-seeking son of wealthy Edwin Alva uses gas from the Big Bang to grant himself various super powers so that he will gain attention from his dad as a supervillain. At the end of the episode, all of the containers for the gas pop at once, which causes him to transform unpredictably- the final form he takes is some sort of horrible, one-eyed, tentacley thing, but shortly afterwards reverts to normal and turns to stone.
Which only makes Alva's earlier remark about wanting a statue creepier.
In an episode of Teen Titans, Beast Boy is infected with a virus that turns him into a werewolf like monster. When he transforms for the first time he reacts like it's very painful.
When Red Star transforms involuntarily. Every molecule of his body going from solid to liquid to gas to plasma in a nanosecond.
In the original Transformers, this happens when Scourge tries to use the Matrix. His body develops grotesque bulges and warts and he falls to his knees screaming in pain as tears stream from his optical sensors.
Cheetor's transformation in "Feral Scream Part 2". It's slow, it explodes from pieces on his body, it's crazy, he has a nightmare about it... and he gains enough power to beat the crap out of EVERYONE when he's feral.
Blackarachnia's unnerving mutation from a promising young Autobot into a techno-organic freak in Transformers Animated.
Turbo Teen. The main character's face stretches out to become the grill of the car, his hands and feet become wheels, etc.
In the world of Codename: Kids Next Door, chicken pox works a little differently than it does in the real world. The victim does indeed get raised sores all over her face and body... except the sores are tiny, living chicken heads bursting through the skin to wriggle and cluck wildly. It's intended as a Visual Pun, but plays out as pure horror.
In Buzz Lightyear of Star Command, Ty Parsec is bitten by "energy vampire" NOS-4-A2, and the radioactive moon on the planet he's stationed on causes him to transform into a "wirewolf", which is a werewolf cyborg. The very first time Ty transforms, wires starts streaming out of the wound on his arm, and attach themselves to his neck. His eyes then grow to large, glowing red orbs, his jaw is stretched into a metal muzzle, and he grows a tail. It doesn't help matters that, from all his groaning and screaming, it looks and sounds exceptionally painful. As the episode progresses, his transformations get less and less creepy, but that first one always sticks out in your mind as pretty bone-chilling.
Exo Squad gives us Automutation Syndrome, which causes the sufferer to slowly melt into a puddle of goo.
What happens to Rourke after he gets slashed by a crystal shard in Atlantis: The Lost Empire. Quite possibly one of the rarest (and most traumatic) examples in Disney film history.
In Happy Tree Friends, this happens to Cub in the Halloween episode "Read 'em and Weep". A cursed book warps the toddler into a tentacle monster.
An episode of SpongeBob SquarePants has Sandy's teleportation device screwing up and fusing Spongebob and Squidward into a weird mess. Then they later get un-fused, then re-fused with every other main character into a blob with people's heads sticking out of it.
The episode with the "small ray" takes Body Horror to a new level; when trying to reverse the ray, SpongeBob puts Squidward through an increasingly painful series of agonizing morphs — catching fire, being cut in half by scissors, having no skin... the worst transformations happened off-screen, with hideous gurgling sound effects that left the true horror to the viewer's imagination, which only made it worse.
"The Ick". It's a virus that causes green ooze to grow all over the victim's body. It's contagious on both touch and ingestion. Somehow, Squidward doesn't realize he's scratching a Ick-y spot, and a restaurant full of diners don't notice they're eating it. In the real world, "Ick" is a protozoan parasite of fish that encysts itself in the skin, causing sores that can eventually be fatal.
In the "strike" episode, SpongeBob is stuck to a window. When Squidward tries to pull him away, he ends up tearing his entire face off and you can see his internal organs.
There's a rather gruesome scene in "Plankton's Regular", where SpongeBob is lying in front of the Chum Bucket's door, and the regular customer opens the door over him, scraping off his face in the process. It doesn't get really disturbing until he sits up, and you can see all the (rather detailed) muscles of his face exposed. * shudders*
There's also "I Was a Teenage Gary" where Spongebob and Squidward accidentally get injected with snail plasma and turn into snails.
Also, the episode where jellyfish jelly becomes a popular condiment on Krabby Patties. Mr. Krabs captures a bunch of jellyfish fish so he can mass-produce the stuff. His factory jelly-extracting equipment is absolutely brutal on the poor jellyfish.
One episode was based almost completely around SpongeBob and Patrick watching a Mermaid Man and Barnacle Boy episode in which they fight all of the show's villains. In it, The Atomic Flounder uses his Atomic Touch ability, which causes Barnacle Boy to sprout a third arm from his shoulder, which proceeds to beat him up. Once he's knocked out, the hand then deforms and sprouts a face, before proceeding to take over Barnacle Boy's body.
The episode "The Inside Job" is full of this trope. Losing sight and hearing, a complete version of SpongeBob's house made of brain including Gary, and two scary transformation sequences.
"Krabby Patty Creature Feature". Jesus Christ. The episode involves a new genetically-altered Krabby Patty formula that turn all who consume it into monstrous Krabby Patty hybrid zombies (and in brutal detail in the case of Mr. Krabs and Sandy), who multiply by FORCING THEIR VICTIMS TO EAT THEM. And somehow this episode wasn't written by David Cronenberg.
The ending of "Teacher's Pests", where after being caught in an explosion SpongeBob, Mr. Krabs and Plankton are sewn together. Namely, Mr. Krabs has been reduced to a head and has lost one of his eyes, which Plankton's body is now replacing, and he's sewn to SpongeBob, who has lost the top of his head where his eyes were.
In "Sing a Song of Patrick", the song that Patrick wrote is so bad that when he and SpongeBob play it for the head of a radio station, it turns his ears inside out. Though it happens offscreen, so we'll have to take SpongeBob's word for it.
In "Ink Lemonade", Patrick tries to scare Squidward into excreting ink so Patrick can sell it as lemonade. The first time he does so by rolling up his skin revealing his organs and talking to Squidward moving his intestines as lips.
On The Fairly OddParents!, Elmer has a boil on his face that has a mind of its own, and it's an evil mind. Yes, it's played for laughs. But it's pretty disturbing if you think about it.
This also happened with Chris' zit on Family Guy, which actually held him hostage for a while.
Also on Invader Zim, though the boil didn't come alive Zim did paint a face on it, and it did hold everyone's attention long enough for Zim to attempt one of his master plans.
The EVOs from Generator Rex are made of this. Nanites are inside every living thing, and can turn it into a horrifically-mutated monster at any moment, at random.
There's also the episode where Rex absorbs too many nanites, which leads to him losing control of them...which, in turn, leads to horrible metallic growths with a mind of their own.
Body Horror is a major thematic element of this show. It's not a cartoon for the squeamish.
In the Legion of Super-Heroes cartoon, Timber Wolf undergoes a forced transformation into a werewolf-man, which also gives him super powers (at least one instance showed his spine starting to protrude from his back and his feet normally has 5 toes but when he's in his feral form they're down to 3). The others experience some form of this, such as Bouncing Boy being splattered against a wall (don't worry, he lives) and them being transformed magically into monsters by evil wizard Mordru.
Inhumanoids was full of it. A lot of it from the hands of D'Compose, who himself kind of qualifies, being a skull-headed dinosaur-creature whose chest has rotted away, revealing his ribcage and his rotting lungs & heart under the ribcage.
Within the first three episodes, Sandra Shore, The Chick of the team, receives the "Touch of D'Compose" and is turned into a hideous giant undead monstrosity. The same thing happened to her again in the eleventh episode.
In episode 7, Doctor Manglar dissolves on-screen when he falls into swamp water contaminated with toxic chemical waste. In the next episode, D'Compose revives him as Nightcrawler, a monster that can be called "human-looking" only in that it's still bipedal.
The same episode also has D'Compose creating an army of undead teenagers.
Adventure Time . The show has several of these, but the most prominent would have to be in "City of Freaks," when the Magic Man turns a bird inside out, organs and all, and it attempts to clumsily fly off, eventually just bumbling it's way along. And it's never seen again.
Simon Petrikov's transformation into Ice King. "As you can see, my skin is starting to turn blue. My body temperature has been lowering at a supernatural rate, to what is now about thirty degrees Celsius. I don't know when it will end... I'm really scared."
The animated G.I. Joe movie has a lot of this. We've got Cobra Commander being revealed as an already somewhat disfigured snake person with multiple eyes (bonus points to the cartoon for hinting at it when Destro was taken aback when walking in on the Commander eating in one episode) and mutated further, first bursting out of his uniform as his limbs start to bulge and fail, then into a large humanoid snake with vestigial arms, then into a human sized snake, a hidden civilization based on particularly nasty organic technology, spores that look to rather painfully mutate all of humanity. Oh, and "Onccccce... wasssss a man..."
Gene-slamming in Street Sharks is rather nasty, what with growing the extra limbs and the screams of pain and whatnot. Bonus points to the first transformation, when the one guy's hand starts to change before the rest of him does.
Savage Opress undergoes a ritual that causes his body to mutate into a much larger and more ferocious looking form. What makes the scene horrific, however, is that you can hear his bones extending, and see his horns grow longer out of his skull. It's a good thing he was unconscious at the time, because one gets the impression that he'd be in agonizing pain if he had to go through all that while conscious.
Darth Maul's condition in "Brothers". His missing lower body has been replaced with a crude, spider-like apparatus, his horns have tripled in length, and there are veins visible all over his body.
Obi-Wan's transformation into "Rako Hardeen" involves his skin visibly warping, and his skull reshaping itself to create his new face. Judging from his reactions, the procedure was very painful.
Phineas and Ferb is usually a very tame show as far as nauseating jokes are concerned, but the episode "Canderemy" is an exception. In a nutshell, Doofenshmirtz builds an inator that combines two objects together. Candace and Jeremy were lucky enough only be attached at the hip. Everything, and everyone else? Not so much◊.
One prominent episode is "Wonder Fun Meat World". Said meat contains an alien parasite which causes anyone that eats it to horrifically bloat, as well as becoming addicted to their food. Also even a tiny bit of said meat can become a Blob Monster to take care of anyone who knows too much. As if that wasn't enough, the process to reverse the main character's transformation goes a bit wrong, causing his face to temporarily melt in multiple ways. The villain is also a 30 foot tall gangsta' colon; the episode managing to be both pure Body Horror and incredibly silly at the same time.
The power of the Dark Ones can also have this effect. Too much exposure to it transformed the kindly Alchemist into the biomechanical Big Bad, and then there's Mandarin who was Swallowed Whole by one of said abominations. He eventually got better, but the combination of said energies and pure stomach acids were not kind.
Parodied in The Mask. Stanley is in Dr. Pretorius' base, having found out that the madman has kidnapped the mayor. On a workbench, we see a red lump of something with a glass dome over it. Stanley immediately exclaims, "Mayor Tilton!", and an exasperated Pretorius says, "No, that's hamburger meat!" Crisis averted...until Pretorius turns the uncooked hamburger meat into a living, snarling monster.
Gravity Falls gives us one in the episode "Double Dipper". Dipper finds a copy machine that lets him clone himself. Unfortunately when he makes the fourth clone there's a paper jam. The result is a clone of Dipper that looks folded and creased like a piece of paper and can only speak pained-sounding gibberish.
Mutagen Man, here, is arguably more horrific than his later 2012 counterpart, being a mess of human organs - brain, staring eyes, chattering teeth, guts - held together by a life support mechanical suit.
The focus of the Teenage Mutant Ninja Turtles (2003) episode "Insane in the Membrane". After four seasons of losing body parts, Baxter Stockman finally obtains a new body via cloning. Soon enough, however, he discovers that it's unstable, as his limbs start deteriorating and melting off, ending with the result seen in the former trope picture. Worse still, his mind is going with it...
Turtles Forever took the mutagen from the original series and played it much straighter, with most of the new mutants in the 2003 'verse suffering involuntary transformations involving a lot of hissing ooze and screaming. Hun in particular ends up Ax-Crazy and vengeful after becoming a turtle.
Then there's the Rat King (AKA Dr. Falco). His body is emaciated and covered all over in horrific burn scars, his eyes are sunken and his teeth are crooked and yellow.
Mutagen Man is a Blob Monster who Was Once a Man, but is now an amorphous translucent green mass of slime with human organs floating around inside it.
The season 2 premiere "The Mutation Situation" sees April's father Kirby transformed into a bat-like creature. What makes it even more frightening is his left arm (which was gripping April to protect her) begins bulging before Mr. O'Neil topples off the roof and into a flock of bats.
Derp April...nuff said.
In Sym-Bionic Titan, the Mutraddi Xeexi forces itself down its victims' throats to make them tell the truth, often exposing their veins. In "The Demon Within" Ilana becomes infected by a Muculox, transforming into a Mutraddi similar to him.
In one episode of The Powerpuff Girls, the Professor's old colleague, Dick Hardly, obtains a vial of Chemical X and uses it to produce and market factory-made clones of the Powerpuff Girls called the Powerpuff Girls Xtreme. This doesn't sound so bad, except that apparently whoever was actually making them didn't do such a great job. Some have eyeballs for heads, one has legs where her arms should be, one has her face upside down, several have horrifically distorted head shapes, and so on and so on. Granted, some of them got lucky and didn't turn out too awful, just being limited to mix-and-match combinations of accessories from their predecessors, but still...yeesh.
In the reboot episode "Bubbles of the Opera", the title character becomes a lovely lady after (1) a dim-witted woman—who gave Bubbles a bad school photo—gives her a bad haircut, (2) gets an allergic reaction from a recalled makeup set and (3) losing a few teeth from hitting the ceiling fan.
Steven Universe, while practicing his shape-shifting powers, turned all his fingers into cats. It was played as a joke initially, but then the rest of him started turning into cats... The result? The show's Nightmare FuelandTear Jerker incarnate.
Amethyst's shapeshifting also goes awry due to a cracked gem in "An Indirect Kiss". At first it just switches the place of her head and one foot (largely comedic), but towards the end of it she starts... unraveling.
You ever wonder what happens when you try to force gem shards together? This is what happens in "Keeping it Together", and it makes everything up to this point look cuddly in comparison. Most of the creatures are only two limbs stuck together, but the final creature...And I Must Scream doesn't even BEGIN to cut it!
Even Garnet is visibly horrified, and nearly falls apart until Steven snaps her out of it. This could count as Body Horror too, as she turns white, and her body starts pulling itself apart, leaving a gaping hole in her stomach. After defeating the monster, she begins talking to herself as Ruby and Sapphire are in severe distress.
Garnet (Ruby): So THIS is what Homeworld thinks of Fusion! Garnet (Sapphire): (shakily) We couldn't have known they would do this... Garnet (Ruby): THIS is where they've been! All the ones we couldn't find! They've been here the whole time! Garnet (Sapphire): Rose couldn't have known! Garnet (Ruby): THIS WAS PUNISHMENT FOR THE REBELLION! Garnet (Sapphire): IT'S NOT OUR FAULT!
The Magic School Bus involves this in every episode where the kids wind up transformed into various animals when you consider the fact they have no time to get used to the new form and can most certainly feel their internal organs being re-arranged as it happens.
In "The Saint" Gumball and Darwin participate in "highly experimentally testing" and turn into horrifically mutated abominations as seen in The Last of Us. Especially Darwin.
In "The Parasite", Gumball and Darwin find that Anais has fused with her new "friend" Jodie. It later turns out Anais was the aggressor in their situation when the boys thought it was Jodie, and when they unfuse offscreen they're covered in a green slime.
Ninjago's former-Big Bad-turned-Team Dad Garmadon got hit with this trope the most throughout the series. He was initially struck by magic lightning when he was banished into the Underworld, blackening his skin and hair as well as exposing his ribcage. He would later grow a second pair of arms during his hiatus in season 1 in order to wield all four of the Golden Weapons without being harmed. He then gets possessed by the Overlord and slowly turns into a dragon, at one point looking like an unnatural mix of human and dragon before going One-Winged Angel. Soon after he regained a human appearance, a spell cast by Clouse caused him and many others bearing a specific tattoo to turn into Anacondrai, and unlike the previous examples, this transformation definitely sounded painful. Jay lampshades the frequency of this trope to Garmadon's face, telling him to pick a body and stick with it.
According to Word of God, any human that stayed too long in ChalkZone would transform into a zoner. Considering that real world liquids are shown to dissolve chalk, it would probably be a painful process.
Superjail! Is really fond of this trope, but the episode "Jean and Paul and Beefy and Alice" takes the cake. In the episode, Superjail slowly gets infected by a strange alien virus which causes your body to bleed and grows extra limbs, eyes, mouths, and all sorts of parts. Made even worse when at the end of episode, where it is discovered there's no cure to the virus.
Pickle and Peanut: At the end of "Busted Arm", Peanut (who spent the majority of the episode trying to upstage Pickle, for his broken arm) gets severely injured from the water slide incident. End result: he ends up as only a head, with veins sticking out.
In the ending of the The Nutshack episode "Got surgery?" the gang's cosmetic surgeries fall apart on them, including Cherry Pie's breast implants popping and leaking and Phil's nose falling off. This is all played for laughs, though.