Sleipnir was Odin’s horse, a son of Loki’s, by the great builder Svaldifari.  Sleipnir had eight legs and was the swiftest horse ever known.

“Yggdrasil’s ash is
of all trees most excellent,
and of all ships, Skidbladnir, 
of the Aesir, Odin,
and of horses, Sleipnir,
Bifrost of bridges,
and of skalds, Bragi,
Habrok of hawks,
and of dogs, Garm
Brimir of swords.”

-- The Poetic Eddas, The Lay of Vafthrudnir