The One-Winged Angel is the TV Tropes-popularized term about a villain's (usually Big Bads) surprising capability to transformed into 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 form 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 occured in their new, stronger form, ranging from unable to return to their pre-transformed state or worse, degenerated intelligence.
Las Plagas: Anyone whom infected themselves with Control Plaga variant of this parasite would be guaranteed to accessed more monstrous form.
Dick Hardly: As result of driniking Chemical X, Dick Hardly turned into a humanoid abomination.
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.
Maleficent: Maleficent's One-Winged Angel form is an enormous black and purple dragon that can spew forth green flame. She used this as desperate attempt to defeat Prince Phillip after the latter got through the forest of thorns.
Jafar: He has 2 version of his One-Winged Angel form; one is his giant cobra snake 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 vagely 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 occured 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: Thanks to the power of Triton's Trisula, she gained the access to become colossal version of herself.
Queen Narissa: Narissa's One-Winged Angel form is an enormous anthropomorphic dragon that ironically lack the ability to fly.
Ludmilla: After she steals Baba Yaga's potion from Bartok, she drinks it and transforms into a purple dragon.
Chirin: Due to rigorous train by a wolf, Chirin has become a monstrous being that can never return to his community.
Ratigan: Upon being exposed by Basil, Ratigan was angry so much that he reverted to his feral self.
Hexxus: Hexxus's monstrous form is a fiery skeleton shrouded by pollution.
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.
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
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