Sleipnir, in Norse mythology is the eight-legged horse of the god Odin. He got its name because he "glides along" equally on land and water, as well as in the air.
In the Eddic sources Sleipnir is depicted as the son of Loki with the stallion Svaðilfari. According to legend, Loki had to prevent the timely completion of Asgard's Walls, because an unnamed Hrimthurse, the builder of that wall around Asgard, coveted the goddess Freya as his wife, as well as the moon and the sun, for the completion of the construction work.
Loki, in the form of a mare, kidnapped the giant's stallion, Svaðilfari, who was helping his owner with the work, and conceived Sleipnir with him. Thus the stallion disappeared for a few days and the deadline for the completion of Asgard's wall passed, whereupon Thor, having just returned, slew the giant with his hammer Mjölnir. Loki later gave Sleipnir, to Odin.
Esoteric Meaning of Sleipnir
Hilda Ellis Davidson writes that Odin's eight-legged horse is the typical steed of a shaman, and that on a shaman's journeys to heaven or the underworld, he is always depicted riding a bird or animal.
Davidson further states that while the creature may vary, the horse is usually common in regions where horses are found, and furthermore, Sleipnir's ability to carry the god is a typical characteristic of the shaman's horse.
The controversial religious phenomenologist Mircea Eliade refers to the eight-legged horse as the typical shaman's horse. It would be found, for example, in Siberia and among the Muria, and would be related to ecstatic experience.