Once upon a time, there were two regal sisters who ruled together, and created harmony for all the land. To do this, the eldest used her unicorn powers to raise the sun at dawn; the younger brought out the moon to begin the night. Thus, the two sisters maintained balance for their kingdom. But as time went on, the elder sister became tyrannical, they relished and played in the day she brought forth yet found the younger a more suitable ruler.

The Legend of Solar Flare. i like this, it shows that like other ponies, Celestia has an evil persona