Sleipnir (also spelled Sleipner), in Norse mythology, an eight-legged gray horse, the swiftest in the world, belonging to the chief of the gods, Odin. Sleipnir was the offspring of the trickster fire god Loki, who had for a time assumed the shape of a mare, and the powerful stallion Svadilfaeri.
According to the 'Prose (or Younger) Edda', right after the gods had established Midgard and built Valhalla, a certain builder offered to build them, in three seasons' time, a fortification so strong that it would be secure against giants. In payment, the builder demanded the goddess Freya as his wife, along with the sun and the moon. The gods accepted on the condition that he build the entire fortification in one winter; if, on the first day of summer, anything were left unfinished, the payment would be forfeit.
The builder asked if he could be permitted to have at least the help of his stallion, Svadilfaeri. The god Loki said that he could. The builder set to work on the first day of winter. The stallion proved to be twice as strong as the builder, and when summer was only three days away, the job was almost finished. The worried gods, trying to think of what to do to avoid payment, directed their anger toward Loki, who seemed to be the one who had caused their dilemma. Loki grew afraid and swore he would arrange something that would cause the builder to forfeit. That evening, when the builder drove out for stone with his stallion Svadilfaeri, a strange mare (Loki in disguise) ran out of the woods. The stallion went frantic and broke free. The horses ran around all night, with the builder trying to catch them. Because the builder was not able to work, the fortification was not completed on time. Loki later gave birth to the foal Sleipnir. Sleipnir became Odin's steed, on which he traveled swiftly through the sky and over the Earth.
In another myth, when the beautiful god Balder was killed and his ghost went to Hel, Hermod the Swift borrowed Sleipnir to make the dangerous journey to the underworld to try and bring Balder back. Hermod rode on Sleipnir for nine days, and when he arrived at the huge iron gate that barred entry to the goddess Hel's domain, Sleipnir was able to leap over it..