Allen Walker of D.Gray-Man has a monstrously deformed left arm that can transform itself into a much larger flexible and extendable claw arm, or a uzi/cannon arm. This only gets worse as the series progresses, with Allen's arm transforming into an even more deformed claw, only for him to later be able to remove it from his body and turn it into a sword. And that's just the main character; let's not get started on the various Noahs, Akumas and Third Exorcists.
Mushizo of Ninja Scroll is a deformed hunchback with a hornet's nest growing straight out of his back.
A few of the characters from the series are worthy of mention as well, such as a swordsman who has a hideous parasitic worm living in his stomach that attacks and kills anyone that is about to kill its host.
Shino and the Aburame clan in Naruto turn their bodies into living hives for their Kikkai insects, but are portrayed much more sympathetically, and with only a few tiny holes in their cheeks as the only visible indicator of their trait (they wear rather concealing clothing, so any further changes are unseen).
His eyes are seen from the side◊ in episode 149 of Part I. The manga says nothing either way.
Heck, a lot of ninjas are like this. Kimimaro's blood limit is to weaponize his bones stands out as pretty freaky. But then there is any ninja who can use the 2nd level of the curse seal, that transforms their body into some monstrosity. Or Juugo, the original source of the curse seal, whose abilities are basically shifting his body into whatever the situation at hand calls for. Kakuzu passes into this territory as well, given that he is partially made of freely-controllable string and artificially extends his life by using his opponents like organ banks whenever something grows too old to work, plus there's those... things that he can create out of the string.
Orochimaru transformed himself into a humanoid snake-like being who can extend his body to bizarre lengths(including his neck and tongue), puke swords, eat large objects, body surf and do other nightmarish things. Also, his real form is a big snake made of other snakes with his human head on the end.
Mentuthuyopi from Hunter × Hunter, one of the elite Chimera Ants, has the ability to mutate his own body. There is seemingly no limit to the number of bodyparts or amount of distortion. He does some really cool stuff with this, too; his period as a centaur-thing was impressive.
In Inuyasha, this is Naraku's main method of attack.
In Ranma ˝ the character of Pantyhose (yes, really) Tarou is bad enough with his ability to turn into a giant Bull/Yeti-Thing in his first appearance. Just to show how Bad Ass (or Jerk Ass more like) he goes and mutates his body more to add tentacles. Obviously, this is form is based on the ushi-oni of Japanese myth, but that doesn't make it any less freaky.
In Speed Grapher, one of the Euphoricsnote (people with disturbing sexual fetishes turned into super powers) is a dentist who can make lots of tentacles made out of dental instruments sprout out of his back. Because backs sprouting tentacles and dental tools weren't terrifying enough on their own.
Guyver tends to invoke this trope, especially with the first activation of the eponymous suit.
In King of Thorn, infection by Medusa can grant this to those with sufficient willpower. The good news: you can make all your dreams into reality! The bad news: by having them explode out of your body like chestbursters. Body Horror doesn't begin to cover it.
Xam'ds in Xam'd: Lost Memories. Even the most subdued use of their powers involves transforming an arm into a grotesque alien appendage or blade.
Alucard from Hellsing honestly resembles a Great Old One more than what is traditionally considered a vampire. Appearing as an amorphous blob full of eyes and teeth, he can turn into a stream of blood capable of drowning an entire city. He carries scores of familiars whose body parts he can materialize into form, like deforming his arm into a snout, so in many ways, he's like a human-sized black hole.
And Alexander Anderson, once he plunged Helena's nail into his chest.
In AKIRA, Tetsuo manifests this trope in the Olympic Stadium when Colonel Shikishima tries to neutralize him; the flesh around the stub where Tetsuo's arm was fried off by the orbital laser shoots out in a massive bloody tentacle that attacks the Colonel. But then, later in the scene, Tetsuo gets shot by Kaneda's laser weapon. You see, Tetsuo is in so much pain that this trope suddenly combines with Superpower Meltdown. Naturally, Tetsuo's Lovecraftian Superpower Meltdown is just as bad as you can imagine it.
In Bleach the 9th Espada Aaronerio has resurrection.
Aizen during his fight vs Ichigo gets a gruesome transformation with the skin of his face splits in half
Almost everyone from the Demon World in Wicked City has some kind of Lovecraftian Superpower. Main character Maki's Femme Fatalons is probably the tamest example in the movie, which says something. The only demon whose power doesn't evoke Body Horror is Dirty Old Man Giuseppe who wields lightning instead.
Pride from the manga can turn into a Living Shadow covered in eyes and grinning mouths.
Wrath from the 2003 anime can fuse his body with almost any matter.
Common among chimeras, too. For example, Jerso can immobilize people with his sticky saliva, and Zampano can shoot spikes from his skin.
In Fairy Tail, the Strauss siblings' Take Over Magic has traces of this. Lisanna's Animal Soul Take Over is relatively benign, granting her the power to transform her body parts into that of various animals. Elfman's Beast Soul Take Over is a bit freakier, granting him the power to transform his body parts into that of various magical beasts. Mirajane's Satan Soul Take Over grants her the powers and forms of various demons.
Immortals in Baccano! have the power to "devour" other immortals, acquiring all of their knowledge all the way down to muscle memory. This process is not pretty.
In Attack on Titancertain humans have the power to transform into Titans. It's implied that the main difference between them and the other Titans is that they can control their power.
Elder Toguro of YuYu Hakusho has the power to manipulate his physical form. This allows him to form his body into any weapon imaginable, travel unseen through the ground by liquefying his body, avoid fatal injuries by shifting his organs around, reattach severed limbs, regenerate from any non-fatal wound, and so on. Later on his body mutates to the point where he becomes truly immortal, unable to die of age due to being a demon, and being able to regenerate from anything including injuries to the heart or brain and obliteration.
Younger Toguro bulks up as he uses more power. 100% has him going outright One-Winged Angel; his skin turns grey and his body becomes over-muscled to the point of deformity.
Bobobo Bo Bobobo is about a guy who can use his hair (including body hair and nose hair) to fight enemies.
Variante has the main character obtain a monstrous arm that can mutate further (her arm ate a dog). She uses the arm to fight monsters.
The Pillar Men from JoJo's Bizarre Adventure are Body Horror incarnate. Specific powers vary depending on the person, but they're all shown to be able to contort their bodies far beyond anything the most flexible, double-jointed human could even hope to do, and even crush their own skeletons into dust to slip through tiny spaces (fortunately, they also have an extremely powerful Healing Factor to undo it) and can effortlessly absorb any living thing into their bodies simply by touching them. More specific powers include burrowing inside other people's bodies and controlling them, extending their ribcages to impale opponents, sprouting chainsaw arm blades, rotating their arms like drills fast enough to create powerful whirlwinds and firing Razor Wind from holes in their bodies, and prehensile blood vessels that can inject literally boiling hot blood into people. When one Pillar Man is seemingly killed, his brain is able to escape his body and seize control of another person and when the leader is transformed by the Stone Mask, he gains Complete Immortality and the power to transform any part of his body into any animal, even when said parts aren't connected to his body (for example, he grows bird wings and fires razor sharp feathers at Jo Jo. The feathers then transform into piranhas to continue attacking him.)
There's also the vampires (originally created from humans by the Pillar Men with the Stone Mask as a food source) who get instances of this, such as being able to contort and warp their bodies in numerous freakish ways; one of them has a nest of venomous snakes in his head, another uses uses his hair as a weapon, and first arc Big Bad Dio can [[split open his eyes]] with a thought to shoot out the liquid inside with the power and accuracy of a sniper rifle.
And from the third arc onwards, several Stands have those types of powers (such as a minuscule Stand that enters your brain through the ear canal and wreaks havoc on your nervous system or turns its victims into People Puppets, or one that manifests as a pustulent tumorous growth either on its wielder (which covers them from head to toe ans can be altered to make them look like a completely diffetent person) or their enemy (where it grows into a grotesque homonculus/conjoined twin who then tries its damnedest to murder its host body). Those two examples are just two from early in the first arc to use Stands, and since the freakiness of said Stands only escalates in latter arcs... It's that kind of series.
In the New 52 Animal Man series, Buddy Baker becomes infused with more power from The Red(the metaphysical manifestation of the Animal Kingdom, and the animal equivalent to Swamp Thing's The Green), and his power to use animal abilities is morphed into actually physically manifesting animal characteristics. These transformations are visceral and not pleasant to look at.
The Spike's mutant ability allows him to fire bone spikes out of his body. The film version has him shooting huge spikes out of his wrists. His teammate Phat can fill himself up with extradimensional gunk, growing to qroteque proportions but increasing his strength and durability exponentially.
And of course, there's Wolverine's claws. Which are bone knives that slice through his forearms and hands everytime they extend. Good Thing He Can Heal indeed.
This video parodies the problems that result from having the claws without the healing factor.
And a parody video isn't actually needed. The comics and adaptations make no bones about the fact that having metal bonded to your skeleton and blades made of the stuff stuck into your arms is in fact not actually a good thing. Logan's healing factor is all that allowed him to survive having it done to him, and when deprived of it by power-dampening characters or tech, the adamantium in his body starts to poison him; he'd die without his powers. And at least before the bone-claw Retcon, driving those claws through his forearms and out of the back of his hands without his healing factor is insanely agonizing - surgery-without-any-anesthesia agonizing because that's kinda what's happening - to the point of being in real danger of having a heart attack on the spot. You're expected to kindly ignore all this when he loses his adamantium and writers who don't want to deprive him of his signature weapon say "Oh, he always had 'em, they were just covered with metal." We also learn just how damaging the presence of the adamantium was during this period, though: his healing and animal-based powers were in overdrive compared to how he normally is; apparently, the overwhelming majority of his mutant might is usually not available to him because it's busy keeping the deleterious effects of the adamantium in him at bay.
Even after the bone-claw retcon, it's shown that popping the claws is still very painful. The first time he popped them in Origins, he screams in agony a moment later. By the present day, he's used to the pain.
Predating the Spike is the even more Lovecraftian Marrow, whose bone growths were originally uncontrollable, random, and disfiguring. She would tear them out periodically to use as melee weapons, healing the wounds left behind with her Healing Factor.
And then there's Tusk, who was exactly like this, but also with little Mini-Me sort of things that could jump out of the pods on his back, for some reason.
A oneshot Morlock was named MeMe. He absorbs people into himself physically to get bigger and stronger; at any given point, screaming humans melted into him and each other will be wriggling around on his body, with the limbs and faces of his less-intact victims jutting out every which way. As they shift and more are added, he never looks exactly the same in any two panels.
A former X-Man with the charming name of Maggott had two parasitic slugs that crawled out of his belly and could eat anything. He spent most of his childhood nearly starving to death until his primary digestive system (the slugs) emerged.
Johnny Dee of the 198 isn't a mutant, but his internalized parasitic twin is. It extends poison tentacles from his gut, and makes mind-controlling zombie dolls of anyone whose DNA it eats.
One short-lived member of X-Force, Sluk, (and when we say "short lived", we mean "he was already dead when his team was introduced") had creepy tentaclethings growing from his face and tentacular feet and hands. His teammates secretly hated him for a few reasons. One was that he was only handy in close combat situations and it was difficult to get him into the right spots a lot of the time. Another was that he wasn't exactly Mr. Personality. But mostly they didn't like him because he just looked really, really weird.
Generation X foe Emplate had mouths on his hands that let him cannibalize mutant bone marrow.
Or the time when he fights Miss Arrow. Or, really, just her entire being. Bonus points for being a Lovecraftian horror in the FIRST place, madam.
Minor Spidey villain the Squid can grow multiple tentacles and spew black ink from his skin.
Spidey villain the Sandman can sometimes appear like this, as some adaptations have him using his ability to reform himself to make an arm come out of his head or other limbs where they just don't belong.
The Bride of Nine Spiders from Immortal Iron Fist can summon hordes of spiders from her chest.
Milder example in Fantastic Four: The Thing looks like a giant rock hulk.
The various symbiotes in the Marvel Universe may as well be parasitic shoggoths, capable of forming tentacles, spines, extra mouths, and other grotesque metamorphoses.
Most noticeable is Carnage, a psychopathic serial killer whose symbiote tends to turn into a cloud of barbed tentacles whenever it feels like it. Also, the reason his suit is red is that it's made up of the Venom symbiote mixed with Cletus Kasady's blood.
Of course, in the Spider-Man: Web of Shadows videogame, Spider-Man's old "Black Suit" symbiote also has the potential to turn into this, if you buy all its upgrades. Waves of writhing, black tentacles covering half a city block is just the beginning...
Perhaps most emblematically, Freak. A parasitic tentacle beast lives in her body. While she has total control over its tendrils... ick.
The evil Green Martian D'kay D'razz, introduced in Brightest Day, uses her shapeshifting powers to frightening and effective use via Combat Tentacles and turning her head into a Venus Flytrap. Assuming a monstrous form comes naturally to her, since she's a murderous lunatic.
In the Marvel Universe, the Celestial experiments on proto-humans that created the Eternals and mutants also created the Deviants. While the Eternals are superhumans with the ability to manipulate energy, the Deviants look like horrible monsters straight out of Lovecraft's works. Fortunately, most of them aren't that bad and just want to live in peace.
Deadpool was an attempt to replicate Wolverine's healing factor that went wrong by having Wolverine's DNA injected into him. He had cancer that was killing him already, and they thought the healing factor would cure that. Unfortunately, the healing factor applied to his body AND the cancer cells, so while his superhuman immune system is perfectly capable of killing the cancer, the cancer can recover and mutate just as quickly.
The first three issues of Generation Hope dealt with the manifestation of the powers of Kenji Uedo, the fifth new mutant since M-Day. His power is a sort of "techno-organic" shapeshifting, but that doesn't quite convey how very Lovecraftian they can be (the first image through the link even looks like an obvious Shout-Out to Tetsuo's transformation in AKIRA above).
In The Umbrella Academy series, The Horror is pretty much the embodiment of this. He's said to have several monsters hidden under his skin. Whether or not they're from space or another dimension or something else has yet to be stated. However, according to some people in universe, they're horrible and disgusting.
The Inhumans are a subspecies of humanity that the Kree genetically modified long ago. Those of Inhuman lineage manifest superpowers when exposed to the Terrigen Mist. The lucky ones retain their human forms after Terrigenesis. The unlucky ones don't.
In the same fanfic, Yuuka Kazami is capable of shapeshifting her own body into all kinds of horrid, twisted forms, all with a floral theme. This isn't just vine tentacles and root claws either. She can actually turn herself into Kaiju-sized plant monstrosities and weaponize every single part of her body. This power of Yuuka is justified in that she's actually an Elder God, albeit a very weakened one.
One Naruto fanfic has the titular character taking Poisonous PersonUp to Eleven. His saliva is poisonous (he can change the poison but has to flush his venom sac a few times to prevent cross contamination), he bleeds hydrofluoric acid, sweats nitroglycerine, and pees cyanide. What really sells it however is how he replenishes his fluids from spitting poison or bleeding acid. He manipulates his spilled blood and that of others, causing it to flow into any open wounds on his body as well as his eyes, ears, mouth, and nose. Kiba mentions that he hasn't been able to sleep alone ever since seeing Naruto use an Exploding Blood Clone to take out a group of bandits then absorb their blood.
Engineers from Tokyo Gore Police are genetically modified humans who sprout weapons from injuries they receive. For example, the first one in the film grows a bio-mechanical chainsaw after his arm is cut off.
Seth Brundle in The Fly gains the ability to wallcrawl, super strength, and even vomit a corrosive enzyme to dissolve food (or enemies). Unfortunately, he gained these abilities when he accidentally fused his genes with a fly, and slowly mutates into a grotesque giant insect/human hybrid. Blessed with Suck indeed.
The Brood. While Psychoplasmics isn't necessarily a superpower, the ability to birth homunculi from your traumatic memories who end up subconsciously doing your bidding might be considered useful, if fucked up.
Videodrome, where Max (probably) gets mutable flesh and a giant mouth in his stomach, which can apparently create hand grenades.
In District 9, Wikus is exposed to alien fluid which slowly transforms him (starting with his arm) into one of the aliens. This gives him both an alien's strength and the ability to operate their weapons.
The eponymous Thing from both the 1982 and 2011 movies usually takes pains to hide itself as those it absorbed... but once discovered or when thinking it's possessing the advantage, the fangs, claws, Combat Tentacles, etc. burst out every which way to kill and infest any nearby lifeform it can.
Corrupted spellcasters in The Banned and the Banished are linked to animals, but the link is always perverse in some way. The first in the series gives birth to man-eating spiders, and they scale up from there all the way to twins who grow pustules that explode into rats. The only one who initially avoids this transforms into any animal he wears the skin of, and he later skins and wears one of the protagonists.
In The Abarat Quintet, the Prince of Midnight, Christopher Carrion, has the ability to distill his nightmares in physical form. They come out of tubes set in his skull and into a tank around his neck, cavorting in the fluid he breathes. On command, they can grow to many times their size and attack.
Oddly enough, the kindly, feminine protagonist of Confessions of Super-Mom has a Lovecraftian power—her warped right hand constantly leaks a mysterious living fluid. She can blast it from her hand with incredible force as a way of knocking people over or making holes in weak objects, and she discovers that it can also be used as a stain remover.
Doctor Who has the Slitheen, who can disguise themselves by wearing people's skins.
In Season 3 episode "Earshot", Buffy fears she might get a Lovecraftian superpower after touching demon blood. Instead she got Psychic Powers, though with great power came great suckiness that got her nearly insane.
In one of the Heroes online novels, one man's superpower was to involuntarily grow spikes; he accidentally killed his wife this way, and then everyone in the van he was being transported to prison in (except for a guard who could turn into liquid).
And then, of course, there's volume 3 Mohinder, who uses his own serum to grant him Spider Man-style powers, with the side-effect of growing scales over his skin.
Maya could cry toxic tears.
In another graphic novel, there was a character who breathed out chlorine gas instead of carbon dioxide.
Pathfinders in Farscape can shoot poisonous bristles from the gills in their heads.
Aside from the Corlata's ability to shapeshift in a particularly gruesome way, one of them apparently had the power to exude an explosive fluid from his hands.
The Halosians can animate their vomit into a seperate entity.
When starved, Delvians produce venomous buds from their skin and exhale clouds of paralysing spores.
Longrunning 2000 AD strip Sláine (being a fusion of Celtic mythology, Robert E. Howard novels and the good old fashioned, classic 2000 AD, punk aesthetic) has its eponymous hero (an appropriate fusion of Cuchulainn and Conan, with a punk aesthetic) warp-spasm similarly.
A feature of The Dreadful Secrets of Candlewick Manor, a supplement for Monsters And Other Childish Things. The more powers you have, the more creepy other people find you and the less you can pass for normal. (In the original game, you have a mon companion. In Candlewick Manor, you have an eerie power which (usually) manifests physically, or else mentally or psychically.)
Sorcerers in the game Sorcerer who have Parasite demons commonly have this kind of power.
Many of the mutations mutants and Chaos champions gain in Warhammer and Warhammer 40,000 fit this, usually a physical boon specific to the chaos god they represent. If they are unlucky, however, they might get something like an eye on their navel or an emu's leg.
Hell, an eye or emu's leg is nothing compared to the grotesque explosions of tentacles (with breasts and tongues if we're talking about Slaanesh) that you can get stuck with!
Chaos Marauders, at least in their Age of Reckoning manifestation, use Chaos energies to spontaneously mutate their limbs into a variety of crablike claws and similar weapons.
In fact, a persistent danger to Chaos champions is that their gods will "gift" them with so many of these mutations that their minds and bodies collapse. These are known as Chaos Spawn.
Several Dungeons & Dragonsprestige classes do this. The Alienist is very explicitly Lovecraftian, as it involves abandoning their sanity, summoning pseudonatural beings from the Far Realm, and modifying their bodies with otherworldly effects. There's also the Vermin Lord (who gets covered in bugs), the Fleshwarper (monster flesh grafts for all!) and the Cancer Mage (how does "Sentient Tumour" sound as a superpower?). You can also take Aberrant feats, which improve your body while making you look really messed up. Special mention should be made of the 3.0 Song and Silence Prestige Class "Fang of Lolth", who slowly gives over her body to the image of the above-mentioned Spider-Queen. Unhinged jaws, bug-eyes, hairy limbs, EXTRA limbs. There are also TWO versions of the Pseudonatural Creature template. As well as a Half-Farspawn template. Both of which add tentacles to an existing creature (although, the first is more of a template for creatures native to Lovecraftian dimensions that happen to bear some resemblance to their Material Plane equivalents, rather than a modification).
The core rules have the Phantasmal Killer spell, which essentially makes you appear to your foes as though transformed into a alien monstrosity. If it works, they die of fright.
Another notable one is called the Warshaper, which basically involves taking a character that can change shape in some way, and going nuts with it. Sprouting claws, horns, mouths and spikes at will, being able to double the length of limbs for better reach. Growing more limbs, all of the above at once...
The Expanded Psionics Handbook doesn't have anything explicitly aberrant (besides illithids and intellect devourers), but many of the powers available to the psychic warrior involve sprouting claws and spitting acid. One of the higher-level abilities, Form of Doom, makes the psychic warrior's body stronger and faster, "complete with an ooze-sleek skin coating, lashing tentacles, and a fright-inducing countenance".
Its expansion/followup, Complete Psionics, has illithid feats and the related flayerspawn psychic prestige class, which allows characters to take on mind flayer traits (up to and including brain-extracting face-tentacles).
Fourth Edition warlocks (particularly the Star-Pact variety) can attack foes with writhing tentacles and swarms of crawling unearthly vermin that sprout directly from the enemy's flesh, or simply attack their sanity with visions and apparitions of this nature. Gained, as the name suggests, by channeling the powers of various cosmic God-beings.
The Pathfinder campaign setting offers sorcerers different bloodlines. One of them, the Aberrant bloodline, gives the practitioner slightly 'wiggy' anatomy (which gets progressively more so as he gets higher in level). Starts out with the ability to spit acid, ends with Aberrant Physiology, in which your character's so messed up, he's immune to critical hits.
The Vivisection Alchemist's recommended Discoveries are things like tentacles, parasitic twins, Tumor Familliars, and vestigial arms. And their base skillset is focused on makingfurries from animals who piss them off.
A Forgotten Realms special is the spellscarred multiclass feature, which has all kinds of nasty Body Horror powers. Including unhinging your jaw to take a bite out of your enemies, bleeding on your sword to make it blister with plague, and creating a rope of flesh that binds you to your target so they can't escape you.
Dragon #s 296 and 300 introduced the Monster Cultist prestige classes. Give yourself over to a monstrous god, and you gain the powers of their natural worshippers ... at the cost of becoming more like them. Examples include Sphere Minion (beholders); Illithidkin (mind flayers); Snake Servant (medusas); Waker of the Beast (tarrasque); Faceless Ones (doppelgangers); Deep Thrall (kraken); Shoal Servant (kua-toas); and Tiger Mask (rakashas). Anything that involves shifting your Creature Type from Humanoid to Aberration is probably applicable.
In Vampire: The Masquerade is the Obtenebration discipline, which at a certain level allows the user to make tentacles out of shadows.
Don't forget what Tzimisce can do with their Vicissitude, which allows them to sculpt themselves or others into slimy pus-heaps or powerful mutant monsters, as they see fit. One sourcebook played up the Lovecraftian connections by making it a parasitic virus from the Umbra. This was not well-received and eventually struck out, but the idea remained that Vicissitude wasn't so much a proper Discipline as a virus devised by the clan's antediluvian for some foul purpose. The Gangrel's Protean discipline also had weird shapeshifting, though usually much less spectacularly disturbing.
Fomori have this and The Corruption as pretty much their entire schtick in Werewolf: The Apocalypse: sure, you can get Super Strength, but you're gonna look like a reeeally ugly Hulk for the rest of your life. Or that convenient armor power you just got takes the form of a thick, chitinous exoskeleton that covers your entire body. It doesn't stop there and Storytellers are often encouraged to go further down the Humanoid Abomination route. Justified by the fact that Fomori get their powers from being fused to a Bane on a physical and spiritual level, and that for each Power Point they get at creation, they must take on an equivalent Taint to balance it out.
Also from Werewolf: The Apocalypse, the Black Spiral Dancer tribe of werewolves. While other werewolf tribes get gifts that allow them to talk to spirits or heal with a touch, the gifts of the Black Spiral tribe are more likely to be hideous mutations.
In Hunter: The Vigil, the Cheiron Group's "thaumatechnology" is based on implanting monster organs and body parts into a human host.
The reality deviant worshippers in Second Sight get a whole section of Body Horror-themed features and rituals.
Sin-Eaters have the Caul Manifestation, allowing them to assume a whole range of horrifying forms. Most notable are the Industrial Caul (which allows them to implant objects in their body) and the Phantasmal Caul (which lets them transform into a living nightmare that can induce paralytic fear or outright madness).
And then there's the Centimani of Promethean: The Created, Prometheans who turn their back on the Great Work and embrace Flux. They can buy all sorts of twisted mutations, from tentacles to extra organs to the ability to turn into a puddle of sentient liquid.
Similarly, the Zeky get a whole bunch of unwholesome Transmutations based around atomic energy and its side effects. Three words: Mind. Control. Tumor.
In Vampire: The Requiem, a few of the bloodlines have these kind of abilities. Take the Carnival, whose special Discipline ranges from contortion tricks to merging with another vampire to form a hybrid. Or the Noctuku, who alter their flesh so they can absorb more blood and better digest vampiric flesh. Or the Norvegi, who lack fangs but make up for it by producing bony spines that allow them to feed through their fingers. Or...
In Mage: The Awakening, Mages have Branding Paradox, where the Abyss warps their body in various ways, which can include temporarily granting them strange mutations of their bodies (such as horns, claws, tails, and, of course, tentacles). There is also a Left-HandedLegacy called "the Legion", whose whole shtick is that they give up parts of their body to the Abyss, and receive mutated transplants in return.
The fourth clause of the Contract of Mirrors allows changelings to turn a limb into a weapon, their skin into bark, etc.
Mummies are all over this, able to call up eldritch abominations or turn themselves into avatars for them.
Demons are basically eldritch abominations in human form, and when they need to, they can reassume their true forms.
In the Glorantha setting of RuneQuest, Chaos Features (powers granted by exposure to "primal Chaos") often have a physical side effect like this—But some folks get Chaos Flaws instead, all side effect and no power.
Nightbane. A Palladium horror dark fantasy, where your Blessed with Suck powers are to transform into an inhuman, grotesque, powerful form.
Exalted has a fair number of horrors with these abilities. Chimera Knacks for Lunars, some of the freakier Yozi charms for Infernals, the Abyssals and their freaky undeath powers...and let's not even get into a discussion of the Wyld. (With the Broken-Winged Crane expansion, Infernals can even literally turn into a shoggoth.)
Voidtech Charms for the Alchemicals generally appear as a techno-Lovecraftian device made of equal parts rust, meat and scientific blasphemy.
3e Infernals are slated to have some new weird things, like the prerequisite for Mind-Hand Manipulation being a brain tumour.
You can learn the Celtic "warp-spasm" in Scion. The picture that accompanies it shows someone in the middle stages of transformation, and it isn't pretty (he's effectively turning into a mutant crow).
One of the many consequences of taking high levels of Taint in White Wolf's Aberrant. Low-level aberrations might include glowing eyes or bulging muscles, while the higher levels of aberration include becoming too hideous to view or having an entire vestigial body. By the time of Trinity, set in Aberrant's future, as far as most people are concerned, all Aberrants are mutated, twisted freaks... though as the game goes on, it's revealed there are a good number who still look pretty human.
In general, Point Build System games such as Champions or GURPS give a player the option of building a character with a repulsive appearance in order to gain points that can be applied to extra powers.
GURPS lists the Battle Jaw, Tentacle Transplant and Ripsnake as potential body modifications, which can come as quite a shock to the unsuspecting.
Twisted Adepts from Shadowrun have embraced the dark side to gain more points to spend on powers. The effect also causes physical mutations and the game suggests that these mutations should represent the powers gained, with an example being supernatural toughness coinciding with growing scales.
Pandemonio's Sacraments, in large part, which let you do things like summon tentacled horrors and giant worms; cause a vicious, gigantic toothy grub to manifest by tearing itself partway out of your stomach; or rip off your skin like a tear-away dress to reveal a demonic form underneath. Blasphemies are nicer in this regard, but still let you do things like fuse people into a single grotesque entity, turn your hands into crab claws, or literally turn your guts into snakes and eels... and it's probably better not to describe what exorcisms tend to entail, though the fact that they're solely the purview of a class of magic known as "Blasphemies" is probably a hint they're not pleasant.
The viruses and the Las Plagas parasites from Resident Evil usually spawn such mutations in those they infect. Even villains who don't go full-on One-Winged Angel tend to have giant claws (Krauser) and Combat Tentacles (Saddler before fully transforming).
As Albert Wesker, who himself gained quite a few of these powers, revealed, this was the ultimate intention of the plague; to transform select humans into superpowered monsters and kill all others.
The hero of FPSThe Darkness, Jackie Estacado, sprouts a pair of snapping serpentine demon-heads from his shoulders, as well as producing dark tentacles to impale foes and destroy walls as needed.
In the first Baten Kaitos game, Geldoblame is transformed into a hideous deformed monstrosity after infusing himself with the power of Malpercio. Jiggle Physics are involved, which is as unpleasant as it sounds.
All of the Vigors in BioShock Infinite have shades of this, most notably Murder of Crows, which works similarly to Insect Swarm above and causes Booker's hands to sprout dark feathers and talons, and Undertow, a Mind over Matter-themed Vigor that causes Booker's forearms to develop cephalopod-like suckers.
Similarly to Warhammer and Warhammer 40,000, ADOM features various "corruptions" that you eventually gain after enough exposure to the forces of Chaos (from traps and Chaos beings, but also simply being deep in a dungeon). These most commonly take the form of Lovecraftian superpowers - although as "superpowers", they're mostly examples of Blessed with Suck. Also similarly to the Warhammer games, taking enough corruption will turn you into a "writhing mass of primal Chaos".
Final Fantasy XI has this in Blue Mages and the backstory's precursor experiments toward creating the Blue Mage. The original experiments included grafting monstrous appendages and material to people, which gave them power but either drove them insane, or transformed them into flan (spell-casting blobs) or soulflayers (read: D&D's mindflayers with the serial numbers barely filed off). When they attempted to just graft a portion of a monster's magic and spirit to the experiments' subjects, they created the first viable blue mages, but even then, a Blue Mage gains their power from assimilating their opponents, and if one pushes too far, has the risk of becoming a soulflayer as well. Which brings some Fridge Logic horror. Just imagine how many players have unlocked Blue Mage on any given server...
In fact from the combination of the danger of their powers, and the risk of being killed by allies who believe they're close to the turning point, it's said that there has never been a blue mage who died of natural causes.
Saki of Sin and Punishment, as a result of absorbing Achi's blood (she turns out to be an Eldritch Abomination, incidentally), gains the ability to transform into a really scary-looking Kaiju that can teleport, shoot giant lasers out of its claws, and grow to such immense size that it is fully capable of battling entire planets. As long as he's paired with Airen, he can control it. If not...it's not pretty. After it activates, even in his human form, he has disturbing looking patches of alien flesh on his body.
Baldur's Gate. In Shadows of Amn, the main character eventually gains the power to transform into the Slayer, a giant demonic creature with huge claws that's nearly unstoppable. Unfortunately, there are side effects.
Dead Space: Unitologists believe that the power of the Marker will allow them to become a new, immortal form of life... and they're not wrong. It just so happens that this "immortality" involves losing your mind and having your body transmuted into a specifically-formed zombie with no purpose than to harvest biological matter to propagate your species.
Some lucky Infected in Left 4 Dead gain considerable powers, at the expense of their... human appearance. May not apply because you can't "Choose" to become a special infected, but the theory is there anyway.
Actually, the only ones that are NOT examples are Parasoul, Cerebella and Valentine. Peacock was once a mutilated girl turned into a cyborg. Regarding Painwheel...well, an image speaks a thousand words◊.
Nero Chaos in Tsukihime is a magus whose entire body consists of the loosely drafted together biomass of 666 wild animals, beasts and legendary monsters that he can either send out in their true form or, more in tune with this trope, use partially, such as generating black tentacles, fangs, claws and such out of his flesh.
After Rose's prototyping takes hold, all the game's enemies (including the Royalty of Derse) gain prehensile chest-tentacles. This is also a natural consequence of the prototyping of Glyb'golyb in the trolls' Session.
Dagon, another Eldritch Abomination, has fused itself with a human being whose body was transformed into the spitting image of a Dungeons and Dragons mind flayer. The tentacles that have replaced his mouth are not just for show; they can inject a rotting poison into his victims.
The Swarm can transform herself into a horde of carnivorous cockroaches. The very thought of having this power has driven her stark raving mad. She's not a cannibal serial killer. Similarly, Hive can transform himself into a swarm of wasps, but has taken it a lot better than The Swarm has and uses his powers to fight crime.
Bloodstone of the Sinister Circle is a "hemokinetic". That is, he has telekinetic control over blood. His usual first move in any fight with a hero is to gather up some nearby blood to use as a weapon, and he gathers it from any nearby innocent bystander who is unfortunate to be close enough to become an unwilling donor.
Ruby Quest, quite literally, in the form of "the treatment". Apparently a universal panacea (even for death), it causes ongoing mutations and loads of Body Horror, which give its victims powers as a result (for instance, a third eye with some sort of true seeing in Ruby's case). Apparently the use of it also allows an Eldritch Abomination called "Cjopaze" into this world, though that's never made quite clear.
Carmilla in the Whateley Universe, since she literally is a Humanoid Abomination: one of her grandparents is Shub-Niggurath, on on her mother's side she's directly related to Cthulhu and the Deep Ones. And you thought your family was freaky. She's got the Combat Tentacles and wierd shapeshifting down. In one story, she split her face open to reveal what it looked like inside, and scared a superhero so bad he wet himself.
A lot of Whateleyites have such powers, due to the Mythos nature of the universe, not to mention plain(!) old mutations. Tennyo's is subtle, but she's no longer remotely human due to channeling the spirit of an ancient construct designed to destroy (or EAT) Great Old Ones, has (occasionally, when she's mad) anti-matter for blood, and even when her powers are negated, can inspire pants-crapping terror merely by making eye contact. Or Fey, who is the reincarnation of a GOO-level elven sorceress, and is supernaturally, mind-controllingly pretty, and can throw around spells that other mages can't even learn. Or more mundanely, Tool (now Demona) who used to have a body in constant flux, which occasionally reacted to his impulses by sprouting erections all OVER! Or any mage who decides to play around with GOO powers. Or any number of GSD sufferers, who have all manner of horribly inhuman transformations to contend with, up to and including And I Must Scream levels, like Puppet, whose blood is so horribly toxic it KILLED HER, and she now lives by occasionally-malfunctioning mad-science. Or... well, most of the Thornies, actually. Or the Foob, who got Mythosed fighting off something we've yet to find out about, and turned into, essentially, a mini-Cthulhu, which dramatically enhanced his psychic powers, rendering him basically unable to tune out the mind-numbing horror people experience when they see his physical form. Whateley's pretty screwed up in places.
Worm: Night (of the villain duo Night And Fog) turns into ... something incomprehensible, fast and super strong as long as nobody can see her.
The effect Rachel has on her dogs counts as this as well.
Echidna definitely counts. She absorbs any dead tissue she encounters into her body, which has accumulated over years into a massive amalgamation of jumbled body parts below her waist. When she comes into contact with something that's still alive she can create copies of it that are always distinctly wrong in some way, and homicidally insane to boot. And if she copies a parahuman, the clone will have their powers or a similar variation on them.
Throw a rock at the SCP Foundation and you'll probably hit something of this nature.
The Big Bang from the Static Shock animated was a Mass Super-Empowering Event that gave all nearby characters varying superpowers. The Metabreed was a gang of these characters who banded together specifically because they were the unfortunate characters stuck with Lovecraftian Superpowers.
In Ben 10: Ultimate Alien, Osmosians have the power to absorb matter and energy and use it to transform their bodies. Absorbing energy has the nastiest effects and warps bodyandmind. This Retcon explains the complete overhaul of Kevin's personality and powers between the original Ben 10 and the rest of the series. When we first saw that he'd gone from an energy-absorbing, murderous, sociopath (who'd absorbed the power of the Omnitrix to become a horrific mishmash of Ben's original 10 alien forms, and eventually lost the ability to turn human again) to a gruff but decent guy who can turn into any material he touches. It made no sense, but now it turns out it's that he could always absorb matter and energy; doing it with energy created the Kevin of the original series and once he switched to matter he went back to being the Kevin of the later series, in mind and body. In Ultimate Alien, he's forced to absorb the Omnitrix's power again, becoming a hideous mashup of Ben's new aliens instead of the original ten. (Yes, this makes him Ax-Crazy again, too.)
Come to think of it, Ben himself. He can turn into aliens; initially ten, eventually a lot, potentially one million, nine hundred and three (and counting! He can scan new ones.) Many of them have powers that only work under cartoon physics, and in terms of appearance, the spectrum runs from "Ugly Cute" to "Ghostfreak."note Ghostfreak's on the creepy side to begin with, with the voice, and the way the cracks on his body are actually tracks that his one eye moves along, but what you think is his body is in fact a cloak. Opening it immediately reduces the opponent - sometimes quite monstrous also! — to absolute gibbering, whimpering terror. There are noRubber-Forehead Aliens in this bunch. And except for Ditto, the un-scary forms are the ones with the most Fridge Horror, like Alien Xnote Looks like a man-shaped window into outer space. Able to control time and space at will, but its multiple souls can't agree on anything and so it's hard-pressed to act. Ben risks And I Must Scream condition; what if they never get around to deciding to move or allow Ben to change back? They almost didn't the first time, and he vowed never to use Alien X again. Naturally, he's been forced to a time or two anyway. and Way Bignote Looks like a more alienesque Ultraman. Is powered by some kind of "cosmic energy," so it's not just as strong as its size implies, it's strong enough to destroy whole worlds in one shot. The race's native environment is the inside of a certain kind of Negative Space Wedgie. They're perfectly capable of generating some of their home environment right over your hometown. That would be bad. Not that he'd have to do that to ruin your day; one punch has the cosmic power to destroy things miles away from the point of impact. Mind you, some of this is All There in the Manual and hasn't been displayed in-show. However, it's not hard to imagine what could happen if the force with which he did some of the things he's done, like throwing an opponent into orbit, were simply applied to the ground beneath him. A nuke would be nothing by comparison... maybe his best tricks remain in the manual because destroying continents would be the side-effects of deploying them. If anyone sees Ben in action and their initial reaction isn't to scream, we almost always find out five seconds later that they're either aliens themselves or familiar with them.
If Ben qualifies, then so does Khyber's pet in Ben 10: Omniverse. The Nemetrix on its collar grants the dog-like alien the power to transform into vicious non-sapient alien predators. Predators that happen to be the apex predators from the homeworlds of Ben's aliens. All of them are horrifying monsters.
Alpha from the Men In Black cartoon, due to grafting alien body parts onto himself. Whenever he looks like an ordinary human, you can bet he's really seconds away from sprouting tentacles - and less and less of him is human every time he reappears.
In Wakfu, Rushu in his true form weaponizes Body Horror, being able to sprout tentacles and fanged maws anywhere on his body. During his fight with Goultard, Goultard tried gut-punching him. This backfired when Rushu's torso simply grew a mouth and bit down hard on Goultard's hand.
There's a species of lizard, the Texan Horned Lizard, that sprays blood from its eyes when threatened.
Other species can stab their own ribs through their poison glands and out their sides.
Bombardier beetles can spray a jet of boiling acid from their abdomens.
No, not a jet. A small chemical explosion. Oh, and they don't have an orifice for it to come out of. It just explodes out through their skin (fortunately it's a pretty weak explosion, so they aren't really hurt by this).
Some species of starfish have clams as their favorite food. How do they get to the tasty clam meat inside? By breaking open the shell with a rock like an otter? No! They latch onto the clam, Face Full of Alien Wing-Wong-style and force the shell open! The clam can only watch in horror as its only line of defense is pulled away and the starfish's stomach acids leak inside!
Most (if not all species) of octopuses can: strangle and crush enemies or prey with their tentacles, confuse them by spraying black ink into the water, paralyze them with potent toxins from their beak-like mouthparts, or change their coloring at will. Some species can even change their texture and limb orientation so they look like harmless seaweed or fish. It's like Prototype, but in real life!
The mimic octopus can mimic twelve species, one of them being a venomous snake.
Spiders inject digestive acid into their prey to make it nice and squishy before sucking out the liquefied organs. They rinse and repeat until all that's left are the totally inedible parts
Marine Cone snails possess some of the prettiest shells of any mollusk- they are also extremely effective predators, several species fully capable of killing a human being.
How does something limbless, nearly blind, and slow as a, well, snail hunt fast prey like fish? Obviously by tracking prey by smell and the motion of the surrounding water, and carefully aiming its organic tank turret which fires a harpoon-tooth laced with custom multicomponent neurotoxin cocktails tailored to its current target type, silly.
The venom of the cone snail is being studied for medicinal uses because it apparently hacks the victim's nervous system; certain species produce a cocktail for hunting fish capable of completely paralyzing and killing an aquarium-sized fish in under half a second.
Then they reel in and engulf their prey like a real-life Kirby.