Moonlight Stardust is a Alicorn who was left by the lake when she was very young. Her parents left her there as they didn't want to handle some pony who's unique like her. She was born an Alicorn and they didn't wanna deal with that. A wizard who lives in the Canterlot castle with Luna and Celestia has taken Moonlight Stardust in to live with them. Moonlight learned magic and became Celestia and Luna's student.