Sleipnir = "The Runner." Óðin’s grey eight-legged steed, also the best of all horses. As told in Gylfaginning, Loki once took the form of a mare and mated with Svaðilfari. From this mating Sleipnir was born.
Svaðilfari = a stallion, owned by the giant who planned to build a wall around Ásgarð.