Daybreaker (Mirror-Universe)

Daybreaker was the older sister of Nightmare Moon and the of an unknown version of Equestria. Due to the corruption of Stygian and power of Princess Eris, Daybreaker was reduced to nothing more than a mere pet loyal only to her master. Daybreaker was primitive, savage, bestial, yet retained enough of her ability to cast complex magic spells strong enough to fell her sister.

History
In her youth, Princess Celestia was a target of Stygian, the Pony of Shadows for her "infinite power" of destruction. Considered a "breaker" and "destroyer" of worlds, Celestia was was eventually ripped apart from her family in order to become just that, a vessel to destroy others. Though she didn't destroy the universe like foretold, she became the "pet" of Princess Eris and has guarded her castle ever since.

When Princess Luna and Stygian from the "main" universe broke in ,she was sicked on them and though she managed to demobilize Luna, Stygian managed to save the day.