Phantasmos Flamehoof

Chaos Knight's horse in MLP style.

Phantasmos Flamehoof, otherwise known as the Armageddon, was a horrifyingly powerful earth pony, born alongside time and space. For centuries he reigned with an undying hunger for war and power, able to rival even the legendary Starswirl the Bearded with his devastating power.

For years Celestia and Luna were unable to subdue the Armageddon, only able to weaken him. Finally, on the day of the Hearth's Warming, they mustered up all the power and will they could, warmed up the Elements of Harmony, and confronted him.

They battled relentlessly for a half-hour, slowly weakening one another, until finally Celestia activated the Elements of Harmony and manifested their power, blasting Armageddon with a devastating rainbow beam and banishing him to another dimension for millenia to come.

That's not to say he cannot return...