Svaðilfari

In Norse mythology, Svaðilfari (Old Norse "unlucky traveler") is a stallionthat fathered the eight-legged horse Sleipnir with Loki (in the form of a mare). Svaðilfari was owned by the disguised and unnamed hrimthurs who built the walls of Asgard.

Gylfaginning
In chapter 42 of the Prose Edda book Gylfaginning, High tells a story set "right at the beginning of the gods' settlement, when the gods had established Midgard and built Val-Hall" about an unnamed builder who has offered to build a fortification for the gods that will keep out invaders in exchange for the goddess Freyja, the sun, and the moon. After some debate, the gods agree to this, but place a number of restrictions on the builder, including that he must complete the work within three seasons with the help of no man. The builder makes a single request; that he may have help from his stallion Svaðilfari, and due to Loki's influence, this is allowed. The stallion Svaðilfari performs twice the deeds of strength as the builder, and hauls enormous rocks to the surprise of the gods. The builder, with Svaðilfari, makes fast progress on the wall, and three days before the deadline of summer, the builder was nearly at the entrance to the fortification. The gods convened, and figured out who was responsible, which resulted in a unanimous agreement that, along with most trouble, Loki was to blame.

The gods declare that Loki would deserve a horrible death if he could not find a scheme that would cause the builder to forfeit his payment, and threatened to attack him. Loki, afraid, swore oaths that he would devise a scheme to cause the builder to forfeit the payment, whatever it would cost himself. That night, the builder drove out to fetch stone with his stallion Svaðilfari, and out from a wood ran a mare. The mare neighs at Svaðilfari, and "realizing what kind of horse it was," Svaðilfari becomes frantic, neighs, tears apart his tack, and runs towards the mare. The mare runs to the wood, Svaðilfari follows, and the builder chases after. The two horses run around all night, causing the building work to be held up for the night, and the previous momentum of building work that the builder had been able to maintain is not continued.

When the Æsir realize that the builder is a hrimthurs, they disregard their previous oaths with the builder, and call for Thor. Thor arrives, and kills the builder by smashing the builder's skull into shards with Mjöllnir. However, Loki "had such dealings" with Svaðilfari that "somewhat later" Loki gave birth to a gray foal with eight legs; the horse Sleipnir, "the best horse among gods and men".