Sleipnir is Odin's magical eight-legged steed, and the is supposed to be the greatest of all horses. Loki had hastily agreed to let Hrimthurs (A Frost Giant) re-build the wall around Asgard, in exchange for the Moon, the Sun, and Freya, only he had to do it in six months. The Frost Giant had only asked to used his gray stallion, Svadilfari. Right as the last brick was about to be placed, Loki transformed into a beautiful white mare, and lured off Svadilfari. Loki later gave birth to Sleipnir, the eight-legged steed of Odin, the offspring of the gray stallion Svadilfari and Loki when "he" was the beautiful white mare.
According to the Prose Edda, Loki returned to Asgard and gave the eight-legged gray colt to Odin, telling him that the horse was the swiftest on earth, and could bear Odin over sea, through the air, and to and from the land of the dead. According to Sigrdrífumál in the Poetic Edda, Sleipnir had runes carved on his teeth.