Daybreaker is an nightmare version of Celestia that appears within one of Starlight Glimmer's nightmares.
Considerably arrogant, power-hungry, and destructive, she boasts that she doesn't need Luna or anyone else, claiming 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 can harness the power of the sun just as Celestia can, however she uses these powers in a much more dangerous fashion, which allowed her to almost escape the dreams of Starlight.
Daybreaker is a sentient form, having escaped the dreamrealm with help from Nightmare Moon. However, she is currently trapped in a glass jar, a quarter of her power and size. A time will soon come when she is released, and she will attempt to rule (begrudgingly) at Nightmare Moon's side.