This is Aquamarina, Alicorn of friendship. She got her cutiemark (a fish) by her extraordinary ability of water-breathing and swimming. She originally came from the underwater world of Waterville, but was sent to Ponyville for safety. Waterville was captured by their enemy, who has no name. Since then she used her magic to fake herself to be a unicorn, so that no pony recognised her. But when she met Princess Twilight, she bowed and showed herself up by opening her wings.
@IonaScratch
