Angel: One episode has a surgeon who can detach body parts to stalk people. Illyria is another example, if an unseen one: she liquifies Fred's organs and turns the skin into a shell as Fred was dying.
Animal Planet Heroes: Any of the shows can dip into this trope (embedded collars, untreated festering wounds, filth-matted and/or mangy pelts, near-death emaciation), as do Animal Planet's vet-clinic shows.
Being Human: One of the main characters is a werewolf, and his transformation is lovingly described in the opening narration of Episode 2: during the process, he actually has a heart attack, and liver and kidney failure, as his internal organs change; and he eventually becomes unable to even scream as his vocal cords tear. As the narrator points out, while any other human would quickly die of shock, the werewolf is somehow kept alive and conscious for the whole thing. This eventually leads to his death, when he deliberately forces himself to transform without the trigger of a full moon, in order to fight the vampires who've kidnapped his daughter. Unfortunately, as a result, he only partially transformed and got stuck midway through the process. The normal accelerated-healing that would normally fix the major internal organ failure simply never kicks in.
Big Bad Beetleborgs: The episode "Buggin Out" is a satire of David Cronenberg's remake of The Fly. Flabber brings a picture of a teleportation device to life, tests it, and a bug gets in the machine (a gnat-like monster named Kombat Gnat who has More Teeth than the Osmond Family and the ability to shrink). Flabber slowly changes into the bug thing just like Jeff Goldblum in the movie. First he starts off with Cute Little Fangs, then develops antennae, then a whole row of razor sharp teeth followed by a more bug-like torso and lobster-like claws for hands. He then bursts to reveal the gnat-like monster he's become. What's worse is that the kids have to fight him in order to turn him back to his normal happy phasm self.
The first episode shows Tommy's initial werewolf transformation as being pretty drawn-out and painful. Subsequent ones are quicker and sillier, perhaps because by then he actually is used to the process.
Buffy the Vampire Slayer: Occasionally shows up. Often it's in the demons, but once in a while, human types are seen too.
One example are the fish monsters in "Go Fish", who are members of the swim team, altered by chemicals. Eventually they tear their old skin away to reveal the monster underneath.
Mohra Demon blood has regenerative powers, which was being sold to injured humans. As a result of magic being gonein Season 9, the blood has a nasty side effect of causing unstoppable cell regeneration. Those exposed to it started to grow giant tumors all over their bodies that wouldn't stop growing.
When Warren Mears is brought back in Season 8, he's still without skin. It goes From Bad to Worse when Buffy destroys the Seed of Wonder, wiping out magic and negating the spells holding Warren together, causing him to collapse into a pile of gore.
Doctor Who is one of the progenitors of this trope, but always finds new ways to put an interesting spin on it:
In several Cyberman stories, the Cybermen are said to have replaced their organic bodies with plastic and metal; when injured however, they are shown to bleed white foam, vomit when shot in the body, groan and scream and and writhe in pain. Distressingly human and not like robots at all. Except the Cybus Cybermen from the New Series and some Mondas ones from Series 6. They're so robotic that the body horror element became very subdued.
Anything written by Philip Martin requires the Doctor's female companion to be slowly, grotesquely, apparently irreversibly transformed into something else. And then the stories all tend to be filled with skin-crawling, glorpy creatures, in case the audience is not barfing heavily enough.
Regeneration is actually used as this a couple of times - most apparently in the regeneration from the Fifth to the Sixth, where the Sixth Doctor is shown to react relatively realistically to the trauma of having transformed into a completely different person with a much less stable brain that he doesn't feel belongs to him, with this having long-term effects on his behaviour. The novelisation of the regeneration story goes into more detail about this and adds an anecdote about a Time Lord who regenerated into something so horrible all the Time Lords could do was put it out of its misery. The novelisation of the First Doctor's regeneration into the Second has several pages of Ben watching the Doctor's Painful Transformation, with attention paid to bones shifting and reforming and skin moving. And then, in the new series, the Tenth Doctor seems to feel this way about regeneration and finds it completely disturbing, possibly even worse than death.
Mission to the Unknown introduces us to the Varga plants. Used as watchdogs by the Daleks, these are giant ambulatory cacti with at least a basic animal-like intelligence. They hunt animals - any animals, including humans - and then shoot their spines into them. These spines carry a venom with unusual effects: the victim first becomes paranoid and psychotic, obsessed with killing; then, they transform into Varga Plants themselves. Even killing the host body does not arrest the transformation. Brrrrr.
In The Ark in Space; it's not the gigantic bugs; those look silly. It's what seems to be a man in a sleeping bag covered in green goo, and before that, the person turning into said bag of slop by melting bodyparts is worse than the alien creature it turns into. "The Ark in Space" also has one of the show’s most famous cliffhangers, with Noah slowly removing his hand from his pocket to reveal he's being taken over by... green bubblewrap?
The Master in "The Deadly Assassin" tried to regenerate beyond his regeneration limit. He has become a rotted walking corpse, in constant agonising pain, living only on willpower and hatred as his body continues to disintegrate.
Revelation of the Daleks, has one character reduced to a living disembodied head with part of his brain exposed (and with additional bits of brain grafted on), inside a transparent Dalek and pleading for death, a wish eventually granted by his own daughter. He has done absolutely nothing to deserve this.
The creationist vicar in Ghost Light slowly turned into an ape. Another example:
Light: (holds up a bloody, severed arm) "I wanted to see how it works, so I dismantled it."
The Empty Child and The Doctor Dances gave us a painful transformation into a mindless zombie with a gas mask, with the air-filter forcing its way up through your throat and out your mouth, and your eyes turning into goggles. Ouch. The effect was originally going to be accompanied by a graphic bone-cracking sound, but the staff decided that was a bit too much. They were probably right.
In The Idiot's Lantern, The Wirepulls people's faces from their bodies. You actually see people walking around with smooth skin where their face used to be. This happens to Rose.
The Abzorbaloff's process of absorbing people in the episode Love & Monsters is both scary and disgusting. There's also a Harlan Ellison-esque pavement-person. Interestingly, the Abzorbaloff was designed by a 9-year-old who won a contest. Watching that episode makes one wonder if the 9-year-old was allowed to watch it.note He was, and he was disappointed. His design was for the creature to be the size of a double-decker bus.
The Toclafane, blade-wielding metallic spheres from, were revealed to be mutilated humans from the year 100 trillion. After attempting to find the fabled "Utopia" (a supposed oasis in the dying universe) and finding only darkness, they went insane and slowly cannibalized their own bodies.
The episode involves the Doctor being aged up tremendously, twice. The first time the cruelty and the nature of the transformation leave a sour taste in the mouth. But then the second instance is deliberately framed like a snuff film. And yikes.
In the Series 4 premiere, Partners in Crime, the monsters are the Adipose, creatures born from human fat that, in times of emergency, convert all matter in a human's body to achieve birth, effectively killing them. And they're adorable.
In the fourth series episode Planet of the Ood, the bad guy turns into an Ood. The sequence has him peeling off his skin shortly followed by him spewing up a piece of his own brain! Talk about trauma. On top of that it was a Karmic Transformation.
In the episode The Stolen Earth, Davros reveals just how he created his new Dalek empire... by opening his jacket where we get a lovely shot of the inside of his chest and see his still beating heart.
In The Pandorica Opens, Steven Moffat introduces the not-at-all terrifying concept of a zombie Cyberman, whose various disconected body parts can all move and function in nefarious ways. First, there's the chopped-off arm which can fire its weapon, play dead, and electrocute the Doctor, then there's the severed head, which moves around on the wires protruding from the neck, using them to ensnare Amy — and then, after the dessicated skull falls out, it tries to snatch up Amy's own head as a replacement. Oh, and then the Cyberman reassembles himself.
In The Time of Angels, we learn that the image of a Weeping Angel becomes an Angel itself. You know how your retina forms images in the back of your eye? Absolutely played for horror.
In The Crimson Horror, there's a series of corpses that turn up petrified and bright red. Turns out, it's caused by a poison secreted by an prehistoric parasite, which a crazy old lady has been letting latch onto her chest in a nightmarish symbiotic relationship. This all goes up a notch in the horror department when it turns out that the Doctor underwent the process some time prior and, thanks to his Time Lord biology, ended up alive, red, and half-petrified. When he's discovered, he's only able to gasp horribly and shuffle around like a zombie. And there's also the matter of poor Ada and her incredibly scarred, raw face which was also caused by the venom.
The Teller makes soup out of people's brains. It wouldn't be so nauseating if their brain fluids didn't leak out through their tear ducts afterward, or their heads didn't cave in. (The caving-in part is one of the more unsettling visual effects the show has ever displayed on screen. Especially when it's later revealed that the people continue to live afterwards.)
In her first few moments on-screen, Saibra demonstrates how unpleasant this power can be when she starts to imitate the memory worm she's touching.
Farscape: In the episode "DNA Mad Scientist", Aeryn Sun was slowly transformed into some weird human/Pilot mix. And that's just in the first season...
In the season four episode "Natural Election", a space-travelling plant infests Moya, and by extension, her symbiotic pilot. Pilot is found unconscious, with black creepers spilling out of his mouth, wrapping around his arms and over his console. And then he wakes up... and starts screaming!
Another episode set aside to test how loud Lani Tupu can scream- "Green-Eyed Monster," in which Crais is tortured by Talyn through their cybernetic link: as a result, horrible lesions and wounds open on Crais' body. And judging by the fact that he spent a full minute screaming for someone to kill him, it hurt.
The Plokavian judges◊ in "The Ugly Truth" appear to be suffering from a extremely painful-looking disease that involves open sores, sagging flesh, and oozing bodily fluids. It gets worse when you look at the materials produced on it in the Farscape RPG:
Movement is impossible on your own, period. Your body is on the verge of turning into nothing but pus and liquefied bone. Your lungs are filling with fluid constantly, and you live in your own filth most of the time. You'd beg someone to shoot you if you could speak. Death occurs at + 5 wounds, but the truth is you'd probably die if you were able to stub your toe.
Over the course of Scorpius' wormhole project, several test pilots are melted after flying through an unstable wormhole: in most example, we don't see the process actually occur. However, in the episode "Incubator," one researcher manages to bring her ship safely through the wormhole, only to discover that the shields she used only delayed tissue liquefaction. Before and After shots readily available.
In the first scene of the pilot episode, an airplane full of people falls victim to a flesh-eating synthetic virus. The scene ends with someone running into the cockpit and the pilot turning in horror just in time to see his jaw rot right off his face.
Later in the pilot, Agent John Scott gets exposed to the component ingredients that made up said virus, which ends up slowly turning his skin translucent... and mushy.
Next episode? A woman is impregnated with a rapidly aging embryo, which ends up growing too large and kills her after putting her in great pain.
Some time later, a woman's radiological treatment turns her into a walking dirty bomb, causing blood to pour out of people's eyes and her own head to explode.
A FBI agent ends up with a custom-made parasite wrapped around his heart like a crawling piece of barbed wire.
Scientists get infected with a cold virus that grows to the size of a small rat and tears up their throat as it climbs out.
A computer signal reduces people's brains to liquid.
People get infected with a bioweapon that turns them into spiny rampaging monsters.
In the fourth season, some of the same people volunteer to be infected with the same bioweapon as part of their cult's transhuman beliefs.
A news vendor gets exposed to a protein that causes the skin over his eyes, nose and mouth to grow over and seal shut.
An FBI agent gets attacked by a transgenic creature, and it turns out the "bite" actually gave him a Face Full of Alien Wing-Wong and he has the creature's larval young rapidly developing in his stomach.
The wife of a scientist is infected with a biological agent that gives her a vampiric hunger for spinal fluid.
And in the new season, ex-soldiers are injected with a compound that, when activated by radio frequency, painfully crystalizes their tissue and turns them into bombs.
There's also a guy who, if he touches you at all, will start ugly malignant tumors growing all over your body.
Many forms show up on Hannibal. One murderess - one who thought she was being nice - turned one of her patients into a beehive and sent another out shambling after a DIY lobotomy.
Heroes: In one of his trademark genius moves, Mohinder injects himself with a serum that he believes will give him super powers without testing it. It does, in fact give him super powers, but the cost is that he begins to turn into a half-insect monster. At the end of Volume 3, a reversal serum washes over him curing the mutations but leaving the superstrength.
In Volume Four, Sylar steals the power of Voluntary Shapeshifting, but finds out he got more than he bargained for when this latest powers combines with another superpower he picked up earlier: psychometry or the ability to read psychic imprints of people (like their memories) from objects they have touched. He finds himself involuntarily shapeshifting in his sleep or under stress, even turning into the likeness of his own, dead mother and holding a whole split personality dialogue with himself.
House: Virtually every patient of the week will have some degree of body horror.
Humanology: This science documentary series shows Truth in Television examples and attempts to treat them, such as face transplantations or an Indonesian man whose limbs were completely coated in giant warts.
Kamen Rider Decade: Has begun to pull this once every two weeks, as each world Decade travels to results in him teaming up with the nominal Kamen Rider there, and him using a Final Form Ride card that turns his own ally into some sort of weapon. This is mainly a result of the transformable action figure line, but the problem comes in as said transformations are kept frame for frame from the toys. So super neat panel flipping on the toys results in limbs bending in disgusting directions on TV.
"This might tickle a bit." Yeah, no kidding, Decade. Gaagh!
Shin Kamen Rider: Prologue defines creepy when it comes to Kamen Rider (THE FIRST and THE NEXT pick up the torch). His transformation includes a third eye emerging from his forehead and his lower jaw splitting in half.
Kids in the Hall: Played for Laughs in one sketch in which Kevin McDonald's character grows a beard, then gets very attached to his new facial hair. The beard ends up taking over his body and forcing him to do all kinds of nefarious things.
And then there was the plastic surgeon who was secretly transforming his assistant into a rat. It also doesn't help that he was giving his client such bizarre suggestions for surgery like "one giant eye in your forehead".
Mighty Morphin' Power Rangers: Lord Zedd is missing all of his skin, has an exposed brain, and is probably wearing a full-face mask for a very good reason. Its heavily implied that he used to look like a perfectly normal human before getting hit by the local Mineral MacGuffin's security system.
The Outer Limits (1995): The episode "Quality of Mercy" involved a young female POW in an intergalactic conflict, who is subjected to painful and horrifying surgery with the intent to transform her into an alien. She is already an alien, and is in fact used as a spy to gather information from her human cell mate.
Rescue 911: This show doesn't skimp out on showing the injuries during re-enactments and some can look pretty horrifying. "Bathtub Baby Burn" is a notable example where they actually show the baby's flesh being burned and his skin peeling off as he cries in pain.
Revolution: The heat-ray weapon used by Jan, Rachel's friend in "The Night The Lights Went Out In Georgia." Basically, it's a tightly-focused microwave-y beam that cooks the target. Alive, if need be. And when she uses it on two militia soldiers...hoo boy, is it nasty.
In Salem, Mary has an extra nipple on her leg, which she uses to feed her familiar (this was based on real beliefs that witches did this).
Mary corrupts a young woman's stillborn child to have a monstrous appearance when it emerges from her body with an unsettling splash as she's huddled in a corner.
Mercy Lewis vomits gallons of blood when she sees the accused midwife, followed by several nails.
We later see how the witches are controlling her-there's a snake in her stomach.
In order to find out John Alden's secret through necromancy, William Hooke's face is pulled off his dead body.
George Sibley cuts open his stomach with a broken-off piece of wood to get Mary's familiar out and escape.
Six Feet Under used this a LOT. After all, this IS a show about morticians, and they spend a lot of time talking about and performing embalmings. There are a LOT of really unpleasant things that happen with a body in the first few days after death that a mortician has to deal with to make it look and smell nice for the funeral.
Stargate Atlantis: Sheppard was slowly turning into something akin to the Iratus bug in the episode "Conversion". Eventually had to be sedated as he was getting increasingly dangerous.
Star Trek: Deep Space Nine: Injuries and illnesses to Changeling characters play up the body horror trope. For instance, the Founders afflict Odo with a life-threatening infection in order to force him to return to the Great Link for judgment. Not only does the infection make it difficult for Odo to maintain solid form, but it also makes his humanoid form look diseased.
When Garak tortures Odo using a device that prevents him from regenerating, Odo's appearance becomes horrific.
In "Identity Crisis", Geordi turns into an alien creature.
"Conspiracy" was full of body horror.
The Borg, who do this to everyone they meet.
Star Trek: Voyager: In "Scorpion" Harry Kim gets bitch-slapped by an alien from Species 8472. Alien cells in the wound begin to infest and transform Harry's body, covering his face in strange tendrils. Fortunately the Doctor is able to cure the infection... except for a solitary tendril up Harry's nose. Or so B'Elanna claims.
And in "Threshold", Paris and Captain Janeway end up evolving into salamanders after breaching Warp 10.
Then there are the Vidiians, an entire species affected with a disease that causes hideous bodily rotting and requires them to rely on mass-scale organ theft to stay alive. Their appearances feature such things as a Vidiian using a gun that teleports Neelix's lungs out of his body, or a Vidiian surgeon slicing the face off of a Red Shirt and grafting it to his own to try and get closer to B'Elanna.
Course: Oblivion sees the crew begin to suffer from a strange illness. They soon discover that they, their belongings, and even the ship itself are all in fact merely malleable life forms which encountered the real Voyager and its inhabitants during one of their past adventures, replicating them so perfectly that they assumed their very identities. Radiation poisoning from their warp core causes them all to degrade back into a liquid state, their bodies literally melting over time as they become grotesque mockeries of their former selves. The ship's own disintegration is almost as horrific in its own right as it literally falls apart, going from a sleek starship to a shifting, gooey mess before finally becoming a nondescript cloud of particles in space.
In "Skin" and "Nightshifter," we see that shapeshifters have shed their skin (by ripping it off) in order to change shape, which includes teeth, hair, and other fun things. But the shapeshifters get Played for Laughs (until the end) in "Monster Movie".
The ruguru, a cannibalistic creature that starts out human, before abruptly developing a hunger for everything - including human flesh. The body horror element kicks in when they begin their transformation into their final form as their bones move under their skin, and after they take just one bite of long pig . . . urgh.
Tales from the Crypt: Not surprisingly, this show utilizes this trope in many of its episodes. In "Ear Today, Gone Tomorrow," a partially-deaf criminal gains the auditory system of an owl through a surgical transplant—only to then develop other owlish features, including an Exorcist-style swiveling head, feathers, and a beak. When he grows the beak, his face cracks open like porcelain and the beak bursts out of his skin. It's as unpleasant as it sounds.
In the comic this episode is based on the man gets the auditory system of a bat instead of an owl. Now imagine what would have happened to him if they went with that one instead of the owl. It really depends on the type of bat though.
The episode "Something Borrowed" centred around Gwen's pregnancy with an alien "baby" and her attempts to hide this from her wedding guests, while escaping from the alien mother who wanted to tear her apart to reclaim the child.
"Sleeper": the sleeper agents' right arm has a sort of implant in it which can turn into a huge blade. This happens to one of the sleepers while she's in her husband's arms. The result is pretty horrifying.
"Fragments": Owen's fiancée appears to have (very)-early onset Alzheimer's. Then it starts to look like a brain tumour. It turns out to be an alien in her brain. She dies minutes later.
Season 3, Children of Earth, features the 456 aliens. They use human children as living drugs, a process that stunts the children's growth and leaves them thin, hairless and immobile, but conscious the entire time.
In season 4, Miracle Day, the entire human race becomes functionally immortal, but still capable of feeling pain, getting sick or sustaining injuries. This provides ample opportunities for Body Horror scenes, such as the suicide bomber reduced to a burnt (still conscious) crisp, the woman who gets crushed inside a car, the fact that deformed fetuses can no longer be miscarried or aborted, and the hundreds of Category Ones incinerated alive...
One episode involved a man who could control the cancer that he had in all his organs and tissues (by eating other people's tumors). He used the cancer cells to generate a new body when his old one became too diseased to function. This particular being could also detect any cancer in anyone, and accurately and precisely describe the type, location, size, disease status, and detailed prognosis of the person, apparently by sight alone. He could also create a new body (and apparently control both bodies simultaneously) at will (provided he had enough material, at least), and could (and did, at least twice) kill off one body as a decoy.