Sleipnir is the eight-legged mount of Odin. He is a horse without equal in Norse mythology. His speed allowed him to run across land, water, and even through the sky. Therefore, he is the ideal mount to cross the different realms of Norse mythology. The magnificence of Sleipnir likely comes from his legendary origins.
His father is Svadilfari, a grand black steed with enormous strength and the helpful companion of a giant. His mother was Loki (yep). The god had temporarily shapeshifted himself into a mare before being mounted by Svadilfari. Loki did not have a choice in the matter, it was a life-or-death situation.
Loki later grants Sleipnir to Odin, which can be considered a curious parenting technique at best. The king of the Aesir would ride it during his runs with the Wild Hunt.