Aaahh!!! Real Monsters featured Ickis, a small red monster with an unfortunate resemblance to a bunny rabbit. He also comes from a long line of loomers, meaning he can size shift into a gigantic beast with red eyes and dripping fangs. For the humans he managed to scare, he definitely came across as a One-Winged Angel.
Adventure Time: If you dare press Marceline the Vampire Queen's Berserk Button, she turns into a giant rampaging bat. This is taken up to eleven in the finale, when she uses the power absorbed from the Vampire King's cloud-essence to outright pulp a GOLB-mutated monster that she thought had just killed Princess Bubblegum. Her father, Hunson Abadeer, also has one◊.
Adventures of Sonic the Hedgehog: In the four-part "Quest For the Chaos Emeralds" saga, Robotnik's goal is to collect the four Chaos Emeralds so he can transform into the Supreme High Robotnik and take over Mobius. Although his plans are foiled by Sonic in the firstthreeparts, his plan succeeds in the fourth part, "Prehistoric Sonic", and Sonic, Tails, and their temporal clones have to defeat him.
The Batman (2004): The Terrible Trio use skin patches to administer the Langstrom serum. In the end, Fox (a guy turned anthro fox) ends up turning into a griffin-like monster.
Kevin 11, a boy with the power to absorb energy, absorbs Ben's Omnitrix, and is able to become any one of Ben's aliens and use their powers. In a fit of rage, he mutates into an amalgam form of all 10 aliens, and can't go back to human, leaving him stuck like that for the rest of the show, and making him a more dangerous foe in Ben's next encounters with him.
Dr. Victor in the two-part episode "The Return/Be Afraid of the Dark" is another straight example, an absurdly large NASA scientist at first appearance but secretly a hulking Frankenstein's Monster-esque alien beneath the façade.
The Grand Finale of Ben 10: Alien Force sees it happening a few times, with Albedo's Ultimatrix being able to unleash the "ultimate evolution" of the various alien forms. Ultimate Humungousaur is green and much stronger than the normal version, while Ultimate Swampfire is a tree with blue growths. And then once Vilgax's ship floods, he unveils "his true form" — a giant squid/octopus.
In Ben 10: Ultimate Alien, Kevin absorbs the power of the Ultimatrix to stop an already Nigh-Invulnerable Aggregor from gaining omnipotence, and then absorbs Aggregor's powers, attaining a new one-winged angel form, and becoming incredibly insane in the process.
Captain Simian and the Space Monkeys has "Gormungus", an alter-ego of Gentle Giant Gor. For roughly the first third of the series, pushing Gor's Berserk Button sends him into an Unstoppable Rage. After the malevolent alien Apax captures Gor and decides that the temper and strength make him the perfect gladiator, but that gentle side just has to go, he zaps Gor with a home-made mutagenic raygun. As a result, whenever Gor gets angry, he grows into a massive, more feral-looking gorilla creature, sort of a cross between The Incredible Hulk and King Kong, until he gets calmed down.
Parodied in one episode. While the brothers are watching a scary monster movie, they are interrupted by Human Kimberly at their door with a gift of broccoli. Gordon is fiercely allergic to the broccoli, and he puffs up so bad that he looks just like the monster in the movie. Mr. Blik and Waffle think he is the monster from the movie and run for their lives. When they can no longer run, they are faced with their only option: fight the beast.
Subverted in another episode. The true form of the Banshee is revealed to be a "beautiful" seal woman — depicted as a seal with a dress and long blonde hair.
In Chaotic, Aa'une went from glowing Evil Overlord to a giant monster with several tentacles when exposed to the energies of Lake Blakeer. Then in the next episode he goes down right Eldritch Abomination by growing extra mouths and more tentacles. Promptly subverted when Iparu's shapeshifting power allows him to assume an identical form and match Aa'une blow-for-blow in his powered-up state.
In Cubix: Robots for Everyone, the second main villain, a female Corrupt Corporate Executive who runs Rubix Corp, turns out to actually be an alien and in one episode reveals her true form. Even more squicky is that the alien sounds like a male!
Characters, particularly Batman villains, in the DC Animated Universe like this trope a lot as well:
Clayface... it's easier to say he really liked to turn into horrifying things during combat.
One of the more way-out examples is the Batman: The Animated Series episode "Home & Garden", in which Poison Ivy creates a race of plant people who start out as normal-looking babies (save for the fact that they've emerged from giant seed pods) but may suddenly mutate into huge, green, ogrish thugs.
Speaking of Batman Beyond, the writers liked this trope enough to play it fairly straight in every other episode. The episode "Curse of the Cobra" is a good example; as a matter of fact, it uses LEGO Genetics as well. This is also the series in which the writers admitted that they would blow up the building if they couldn't figure out a good way to end an episode.
Dex Hamilton: Alien Entomologist: When Zap's insect DNA overwhelms his human DNA, he transforms into a gigantic, monstrous insect form. Fortunately, his friends are able to bring him back.
Drawn Together: When the king of Mexico drinks the worm...
Venger, from Dungeons & Dragons (1983), transforms into a monster in the episode "The Girl Who Dreamed Tomorrow".
The Emperor's New School: In one episode, Yzma turns into a giant snake. In another episode, she and Kuzco have a transformation potion duel — one of the things Yzma turns into is a Tyranosaurus Rex. She also once made a potion that would turn her into the most hideous, terrifying thing imaginable, so frightening the other wouldn't be able to do anything about her... comically subverted when the potion does nothing, implying that she already is.
Generator Rex: Justified because the character who goes One Winged Angel is a shapeshifter, interesting because the hero, Rex, is the one who does it, admittedly in a flashback.
In "Dreamscaperers", after Mabel foils Bill Cipher's plans, he turns into a giant, red version of himself that shoots fire and lasers and makes your worst nightmares come true. Yeesh.
Later on down the line, in "Weirdmageddon Part 1", Bill achieves his physical form, which while only slightly different than his original form, he does briefly turn into a prism-like monstrosity. His physical form also allows him complete control over reality. To say it works for him is a huge understatement.
In the series finale, "Weirdmageddon Part 3", Bill transforms into what may very well be his most demonic version of him: while chasing the twins, Bill becomes a three-dimensional, red monstrosity with a yellow hat, several mouths filled with sharp teeth, his arms and legs become yellow, long and spindly like those of a spider, his eyes become black with a yellow, reptilian sclera and he also gains some more too, and he also sprouts some extra limbs growing from his mouths.
Several demons and angels are shown being able to assume more intimidating and less human forms, often when they get angry. A clear example of someone doing it for the Final Battle is in the final episode of the second season, when Vox gets so empowered he becomes the most powerful Overlord and, in his fight with Alastor, assumes a large, multi-legged form resembling a bacteriophage. It's up to guessing whether he could have done just that if he hadn't got an extra power boost.
At 100% approval rating, Vox can transform into his full demon form. He approximately doubles in size, his arms bulk up and tear apart the sleeves of his suit, his back splits open to release a secondary spine that splits into multiple mechanical legs resembling a spider or crab while his actual legs gain jagged edges, a series of external ribs grow around his sides, his left Hypnotic Eye becomes a square that takes up an entire quarter of his screen while he grows an additional eye on the right side of his face, his mouth becomes jagged and his voice becomes distorted, and he extrudes several wires that end in crackling electrical plugs or yet more hypnotic eye screens. He reverts back to his original form once Alastor cuts him in half and his approval ratings start to drop.
Happens a lot in Jackie Chan Adventures. This happens to anyone who wears one of the oni masks, and in the final season it happens to the villain Drago, or anyone else, who absorbs a demon power. When Drago gets all the demon powers, the end result is quite... monstrous. Most people would probably say that Ultimate Power is not worth the price of looking so ugly.
Shows up in The Legend of Korra when Korra's Evil Uncle merges with Vaatu to create the Dark Avatar. After he defeats Korra and destroys Raava, he becomes a battleship-sized Dark Spirit and proceeds to go on a rampage.
In LEGO Star Wars: Terrifying Tales, once Vaneé is finally able to wield the power of the Sith Holocron, he uses it to construct a towering Giant Mecha suit around himself and wield the dangerous Saber of Scardont with full control. In a variation of the trope, he loses the holocron quite quickly, but is still able to hold on to his new powers, though he knows they'll fade in a relatively short time.
In ¡Mucha Lucha!: The Return of El Malefico, after El Malefico is beaten in the first fight, he transforms into a towering giant to continue the fight.
My Little Pony 'n Friends: Lavan, villain of the "Quest of the Princess Ponies" serial, transforms to a powerful crystalline form by absorbing the magic of the six great magic wands he stole from said Princess Ponies. His careless wresting of the power from the wands disrupts all the world's magic, meaning that doom is coming on two fronts.
Emperor Belos looks like a normal man with a skin condition affecting his face, but due to him consuming Palismen to extend his lifespan, his body has slowly started to deteriorate and warp beyond recognition. He's been keeping himself together with magic, but when Luz brands him with a sigil, the Draining Spell drains all of the magic out of his body, forcing a transformation into his true form — a twelve foot tall, skeletal monster made of rotting, liquefied flesh, absolutely covered in glowing blue eyes.
In the series finale, Belos possesses the heart of the Titan itself in a desperate attempt to complete his genocide of witchkind with no one to stop him after learning that Titan magic can cancel the Collector's Reality Warping. In the process, he gains an even more monstrous form in which the flesh-like goop that makes up his body becomes a lichen-like Meat Moss that covers the entire Boiling Isles, and Belos himself transforms into a towering draconic beast rooted to his castle, in mockery of the Titan's image.
Ozzy & Drix: A corrupt mayoral candidate who has been illegally using growth hormones to sabotage his rival's campaign gets caught red-handed with the stuff, downs it in a panic, and you can guess the rest.
A few characters in the series pulled this trick. In the episode "Knock It Off", for example, Professor Utonium's slacker roommate from college, Dick Hardly, gets some Chemical X and starts making hordes of Powerpuff clones, renting them out to big cities as a new crime-prevention system. When the Girls learn about his scheme and confront him, he tries to hide the remaining Chemical X by swallowing it... and turns into a giant monster.
Mojo became one giant ape in The Powerpuff Girls Movie. Him also did this in the episodes "Speed Demon" and "Power Noia", though into two different forms. It actually works very well for HIM in "Speed Demon" and for Mojo in The Movie. So does Sedusa when she tricks the Gangreen Gang into stealing three Egyptian artifacts for her. Not only does it restore her hair, but it turns her into a giant monster, with live snakes for hair, bent on destroying the Powerpuffs and Townsville. Remember, it's always about the tiara!
Samurai Jack; Aku forcibly transforms Ashi into a stronger, faster, demonic version of herself after discovering she's his biological daughter.
Parodied with typical relish in a Halloween episode of South Park. The spirit of Wall-Mart revealed his "terrifying true form"... by removing his stick-on moustache.
Korn (stay with us here) tried to do a heroic version of this during their guest appearance in another Halloween episode. It didn't quite work out. "Korn powers activate! Form of — CORN!!!"
And in "Pandemic Part 2", there was an evil U.S. secretary of Homeland Security who, after revealing he was the leader of the guinea creatures, transformed into a GUINEA PIRATE.
Star vs. the Forces of Evil: The Mewni queens have a special ability to invoke this, turning into super-powered humanoid butterflies as a last resort in life-threatening situations. Star unlocks the ability at the end of Season 2 to escape from the dark dimensional prison Toffee put her in and literally nuke him to death. Star's mother Queen Moon can do this too, and the way she talks about it indicates that it's normal for a queen to have this ability.
Teen Titans (2003): After pissing off Raven, Dr. Light and Terra nearly get their souls eaten, and Dr. Light didn't escape without probably getting a few bites taken out of his.
ThunderCats (1985): "Ancient spirits of evil, transform this decayed form into MUMM-RA, THE EVER-LIVING!!" Snarf proves that anyone can channel the 'ancient spirits of evil' if they're allowed into the transformation chamber. This might seem disloyal of them, but hey, the spirits are evil...
In Todd McFarlane's Spawn, Violator eventually changes from his Monster Clown appearance into a huge horned monster whenever he wanted to smack some stupid out of Spawn.
Transformers: Prime has a human example with Silas. After being mortally injured when Nemesis Prime falls on top of him, his mind is transferred into the body of the recently killed Breakdown.
In the T.U.F.F. Puppy episode "Toast of T.U.F.F.", Keswick invents R.I.T.A. (short for Robotic Interactive Toasting Appliance), a toaster with artificial intelligence who proves she can do much more than make toast. Near the end of the episode, when Keswick tries to unplug R.I.T.A. to keep T.U.F.F. Headquarters eco-friendly, she turns against him and turns herself into a mechanical monster using all of T.U.F.F.'s electronic devices.
Ultimate Spider-Man (2012): In the finale of the 4-part "Spider-Verse" arc, the Goblin infuses himself with the DNA from the various Spider-Men that he's encountered traveling across the multiverse to become the Spider-Goblin, requiring the original Spider-Man to team up with the six other Spider-Men they've met to defeat him.
Also happens to the Trix in Season 2 (the Negatrix), Season 5 (their Dark Sirenix forms, which was a physical mutation instead of their usual costume change upgrades).
Tritannus, the main villain of Season 5, invokes this in the second episode of the season when he absorbs magically corrupted oil that gives him incredible magical power. Not only has he stuck around since then, but the One-Winged Angel form is what he's used for most of the season; he's pretty much useless when in his original body, though he typically has someone to protect him when that happens. His plan seems to be acquiring another such form, which cannot be good.
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