One of my headcanons involves Fausticorn/Rosetta and Discord being twin entities, Creation and Destruction, Harmony and Chaos.
The sister created all the living creatures while the brother corrupted some of them, making the monsters/predators of the world (Chimeras, Cragodiles, Hydras etc.)
She made lands perfect to live in while He made dangerous uninhabitable lands.
While they were close for most of the beginning of their existence, they eventually drifted apart due to their different purposes.
Now He resides on his side of the planet, planning on how to take over Her side.

Fausticorn belongs to Lauren Faust.