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.
Named in honor of the 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.) Video games in general absolutely adore having their final boss do this, even when their original form is scary enough anyway. In fact, it's gotten to be somewhat of an arms race: thanks to the popularity of Frieza of DragonBall Z in 1991, three-form bosses are now somewhat common, and those games going for "epic" will sometimes go for even more. In JRPGs, particularly, it is rather common to see two stages of One-Winged Angel: the "bizarro" form, that is huge and scary, and the "angel" form, that is winged, eerily beautiful and accompanied by ominous Latin chanting. Once beaten, or on becoming even more powerful, the villain may cross the "Bishonen Line" and into safer territory for an "Evil Makeover" to work its magic (though it is a huge case of "Tropes Are Not Bad"; as much as it is used, one would be hard pressed to complain when they turn into something completely awesome). Usually accompanied by the stock phrases "No one who's witnessed this form has lived to tell the tale!", "I have only begun to fight!", or "This isn't even my final form!".
Sometimes you never even fight their human form at all and they immediately turn into a monster. Can count as "The Unfought" if they showed fighting ability in their human form. This is more popular among minor videogame villains who will often transform into tougher versions of earlier monsters like in the Breath of Fire and early Final Fantasy games, as a unique battle sprite for them would take up extra development time and storage space on the cartridge (one unique boss could take the space of several mooks, more with the use of a Palette Swap).
Interestingly, heroes, particularly transforming characters, have been known to occasionally use this "turn into a big scary monster" tactic for their Super Modes as well. It's extremely rare for it not to cross into "Superpowered Evil Side" territory. And for those with firmer morals, this qualifies the villain as a monster, making it fine to kill him.
One way to defeat a villain who became giant is to become one yourself. A fight of giants can be even worse than a single giant, but it may be done anyway if there are no other options left.
Las Plagas: Anyone whom 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.
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: Thanks to the power of 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 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.
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