Princess Luna, known also as Nightmare Moon or Night Mare Moon when transformed or under certain other circumstances,is an Alicorn pony, the co-ruler of Equestria alongside her older sister Princess Celestia (before their retirement in season 9), and the adoptive aunt of Princess Cadance. She also serves as the main antagonist of the season one premiere, where she initially appears as Nightmare Moon. Princess Luna is called by her respective early names Princess Selena/Night Mare in some storybooks.