Daybreaker

The episode A Royal Problem reveals a potential evil alter-ego for Celestia named Daybreaker, self-described as "the better, prettier, and more powerful version" of Celestia. This version of Celestia is considerably arrogant, power-hungry, and destructive. She boasts that she doesn't need Luna or anyone else, claims that she should have destroyed Luna instead of banishing her to the moon, and dismisses the notions of balance and harmony in favor of ruling over all.

Daybreaker harnesses more of the sun's power than Celestia does, allowing her to breathe fire and generate intense, burning light.

Daybreaker briefly reappears with Nightmare Moon in a vision shown to Twilight by King Sombra in The Beginning of the End - Part 1.

Daybreaker appears almost the same as Celestia: a tall alicorn with a white coat. However, unlike Celestia, both her mane and tail are made of fire. She has yellow eyes with dragon-like pupils and black irises, possesses fangs, and her cutie mark is surrounded by orange and yellow. She also wears orange armor on her hooves, chest, wings, and head, with her helmet adorned with a red jewel, and her chest plate has her cutie mark on it.

Daybreaker is incredibly vain about her appearance and abilities, believing herself to be superior to Celestia in every way. She has no qualms about destroying anyone who opposes her, even her own family, and is hellbent on eliminating Luna so she could rule Equestria alone. All of her maniacal laughing is an obvious sign of insanity.

It's safe to assume that Daybreaker's powers are the same as Celestia's, but according to herself, Daybreaker's power is even greater. She can fly, shoot powerful magic bolts, and produce immense amounts of fire to incinerate her enemies. She was able to overwhelm Nightmare Moon, a powerful alicorn in her own right.