Princess Luna, known as Nightmare Moon or Night Mare Moon when transformed or under certain other circumstances, is an Alicorn pony, the younger sister of Princess Celestia, and the main antagonist of the season one premiere of My Little Pony Friendship is Magic as Nightmare Moon. She is also the adoptive aunt of Princess Cadance. Eventually, the Elements of Harmony restore Luna to her former self and she returns to rule over Equestria alongside her older sister. Princess Luna/Nightmare Moon is called by her respective early names Princess Selena/Night Mare in another depiction.

http://mlp.wikia.com/wiki/Princess_Luna