Slepnir is Odin's mount according to Norse mythology. In all depictions, he has eight legs. He is one of three monsters that the god Loki acted as a parent to and is also, by extension, a half-sibling to Fenrir, Jörmungandr, and Hel.
According to the myths, Slepnir's sire was a horse called Svaðilfari. Svaðilfari was in the possession of the man who had requested to build Valhalla in exchange for Freya, the sun, and the moon. At Loki's urging, the other gods permit this if he can complete the entire building in three seasons on his own, a task that they fully expect him to fail at. However, the builder has his horse do much of the work and it becomes obvious that he will succeed at his task. The gods blame Loki and threaten to torture him unless he fixes the problem. To disrupt the work, Loki takes the form of a mare to lure Svaðilfari away, halting the builder's work for the night. When the gods discover that the builder is an ice giant, Thor kills him anyway. Eventually, Loki gives birth to Slepnir, the greatest horse in the world.