Source

She-Who-Devours was the first dragon. Massive and powerful, she settled upon Equis after consuming a nearby star. From her came forth the six dragon breeds. Fire, Air, Water, Earth, Mind...and Leviathan.

Millions of years later, the fifth Leviathan has arisen. Spike, assistant to Twilight Sparkle.

And woe betide those who cross the Leviathans. For they are patient...they are powerful...and they are eternal.