Name: Mysteria (Saecularia)
Parents: Celestia and Discord

Bio:
Born from magic, and as a way to seek revenge, a younger Discord created her using a feather from Princess Celestia and his own chaotic magic to create Mysteria. He originally named her Saecularia which means worldly,(she will conquer worlds!!!- Discord) Back then, a crueler version of Discord needed her to be fully grown, not a tiny powerless cub. So he created a spell and made her fully grown. However, her brain had not grown at the same capacity, so she was insanely powerful, but with the mind of a child. Discord planned to use her when celestia and luna were at their weakest(the plunder vines) but he was defeated before he could summon her. Mysteria roamed the world searching for Discord(even though he as cruel to her, he was the closest thing she had to family) Ponies began to whisper about a "mysterious thing in the dark" and decided to call this dark thing a Mysteria(idk ok) Mysteria soon wandered into the crystal kingdom but was spotted by a guard  and chased out into the frozen north where she accidently, created an ice bubble and froze herself. nopony knows where the mysterious creature is, but soon the story was lost into legend.