The One-Winged Angel is the TV Tropes-popularized term about a villain's (usually Big Bads) surprising capability to transform into a monstrous version of their former self as means to defeat the heroes with full force, normally in desperate situation.
The means to acquire one's monster form would be vary, ranging from using certain dangerous forms of dark magic or certain mutagenic drugs/chemical substances. The effect of the villains' transformation result can give them advantages over the heroes as they predictably become more powerful, though certain flaws would always occurred in their new, stronger forms, ranging from unable to return to their pre-transformed state or worse, degenerated intelligence.
This term is named after theme music and alias of Sephiroth, the main villain and final Boss of the videogame Final Fantasy VII (even he has in fact 7 wings, but just one in place of an arm). Various villains, especially ones in video games media, have access to monstrous form to serve as challenges for heroes and other good guys who sought to defeat them. This even become recurring theme, and to increase the challenge, some villains even have multiple monstrous forms each stronger than previous ones as seen in case of Frieza of DragonBall Z in 1991.
This popular theme is not exclusive to villains alone, as some heroes also have his/her own One Winged Angel forms, but still share same risks and limitations.
Las Plagas: Anyone who have infected themselves with the Master Plaga variant of this parasite would be guaranteed to accessed more monstrous form.
Mojo Jojo: With the help of Chemical X that gave him his powers, he can turned into more powerful being.
Kevin Levin: Kevin can turned into a monstrous hybrid alien should he absorb a portion of Ben's Omnitrix's powers, but this cost him his sanity.
El Macho: El Macho can turned into a purple-furred beast with PX-41.
King Candy/Turbo: Turbo gained all of Cy-Bugs' powers due to devoured by a Cy-Bug that released by Ralph, but the cost was he cannot resist his Cy-Bug instinct to approach extremely bright light.
Hun: In Turtles Forever, Hun gets himself exposed to Mutagen and turns himself into a mutant turtle, the one thing he hates most in the world, due to "mutant turtle filth" being the last thing he touched.
Mohawk: Mohawk drinks the Spider serum and transform into a hybrid form of Gremlin and spider.
Judge Doom: reveals himself as a Toon with burning cartoonish red eyes and a high pitched squeaky voice who killed Eddie Valiant's brother, Teddy five years ago.
Queen Grimhilde: Drinks a magic potion she made to transform herself into a witch.
Maleficent: Maleficent's One-Winged Angel form is an enormous black and purple dragon that can spew forth green flame. She used this as a desperate attempt to defeat Prince Phillip after the latter got through the forest of thorns.
Jafar: Jafar has 2 versions of his One-Winged Angel form; one is his giant cobra form and the other is his monstrous genie form.
Rothbart: Rothbart's One-Winged Angel form of choice is Great Animal, a monstrous large hybrid monster with the head of a wolf, the body of a bat, the talons of an eagle and the tail of a lizard, giving a vaguely dragon/wyvern-like appearance.
Twilight Sparkle: When Principal Cinch forced Twilight to use magic that drained in her device to win he Games for Crystal Prep, a freak accident occurred where Twilight turned into Midnight Sparkle that went on rampage until Sunset Shimmer stop her.
Mad Madam Mim: Mad Madam Mim's One-Winged Angel form is an enormous purple dragon that retained her human hair.
Sunset Shimmer: The result of using Twilight Sparkle's element of magic crown turned Shimmer into succubus-like fiery demon.
Ursula: Due to the magical power of King Triton's Trident, she gained the access to become a colossal version of herself.
Queen Narissa: Narissa's One-Winged Angel form is an enormous anthropomorphic dragon that ironically lacks the ability to fly.
Ludmilla: After she steals Baba Yaga's potion from Bartok, she drinks it and transforms into a fat amethyst-colored dragon.
Dorian Tyrell: After he steals the magic Mask of Loki from Stanley Ipkiss, he puts it on his face and transforms into a Beast like version of The Mask.
Prince Aragon: When he gets angry, his Amulet of Aragon glows and he transforms into a black and purple dragon.
Chirin: Due to rigorous training by a wolf, Chirin has become a monstrous being that can never return to his community.
Ratigan: Upon being exposed by Basil, Ratigan was furious so much that he reverted to his feral self.
Hexxus: Hexxus's monstrous form is a fiery skeleton shrouded by pollution.
Combiner team Decepticons: Multiple incarnations of the Decepticons have displayed the ability to merge with the various members of their group into a single large 'bot. For example,
The Constructicons (Scavenger, Long Haul, Mixmaster, Scrapper, Bonecrusher, and Hook) can form Devastator. Other Constructcons such as Rampage, Demolishor, Hightower, Overload, Quickmix, Buckethead, Wideload, Steamhammer, Bonecrusher's younger brother Gravedigger, and even the Autobots Scoop and Hauler are sometimes involved in various incarnations of the team.
The Combaticons (Onslaught, Swindle, Brawl, Vortex, and Blast Off) can form Bruticus. Shockwave can also form a weapon for Bruticus to use in combat.
The Stunticons (Motormaster, Drag Strip, Breakdown, Dead End, and Wildrider) can form Menasor. Heatseeker, Wildbreak, Slashmark, Offroad, and Blackjack are sometimes involved in various incarnations of the team.
The Predacons (Razorclaw, Rampage, Divebomb, Headstrong and Tantrum) can form Predaking.
The Terrorcons (Hun-Gurr, Blot, Sinnertwin, Rippersnapper, and Cuthroat) can form Abominus.
The Infernocons (Skulk, Rupture, Thrash, Gorge, and Glug) can form Infernocus, as shown in the toyline for Trasnformers: The Last Knight. However, the film itself depicts a total of six Infernocons forming Infernocus. While fans claim that the toyline Infernocons are indeed present in the film, albeit with identical robot modes, the name of the sixth member currently remains unknown.
Now shall you deal with me, O Prince, and all the powers of HELL!
~ Maleficent right before transforming into a dragon
A snake, am I? Perhaps you'd like to see how s-ss-s-snake-like I can be!
~ Jafar while transforming into a giant snake.
ONLY IF YOU DEFEAT...me.
~ Rothbart right before transforming into the Great Animal.
Infernocus, transform! Destroy them! [All six Infernocons merge into the monstrous combiner Infernocus.] (Hound, stops firing his machine gun: I don't have a lot of ammo for that thing.)
~ Megatron orders the Infernocons to combine, the end product of Infernocus being too much for the Autobot Hound to handle, in Transformers: The Last Knight.
Two years have now passed, Chirin has changed from a determined lamb, into a fearsome beast.
~ The Narrator
So then Wolf I am no longer a weakling sheep instead of fangs I have horns which are just as sharp, my hooves are harder that the rock, and it has become my nature to fight without fear of death.
~ Chirin to the Wolf.
We could of ruled together Gru but now... you're going to die
~ El Macho before drinking the purple serum
[The Ninja Turtles and Splinter battle Hun, who has been turned into a ferocious mutant turtle.] (2003 Raphael: So what's with the new look, gruesome?) That Mutagen turned me into the last thing I touched: mutant turtle filth!
~ Hun explaining to 2003 Raphael the circumstances of his mutation in Turtles Forever.
Correction. The new, improved Scrappy. Because I, Scrappy-Dappy-Doo, have absorbed enough energy......to rule the world with my all-powerful army! And I've brought you here, puny, pathetic Mystery Inc......to witness my moment of triumph. All I need to complete my transformation is Scooby-Doo!
~ Scrappy Doo while transforming into a giant monster
I'll give you Chanticleer!
~ The Grand Duke of Owls before growing to ginormous size.
~ The Shapeshifter right before he changed into a horrid monster to destroy Fairy Land.