In Norse Mythology, Svaðilfari (meaning "unlucky traveler") is a stallion that fathered the eight-legged steed of the god Odin, Sleipnir, with Loki (who was in the form of a mare at that time). Svaðilfari was owned by the disguised and unnamed Hrimthurs who built the walls of Asgard.
Early in the beginning, an unnamed Hrimthurs, in exchange for the goddess Freyja, the sun and the moon, offered to build for the gods a fortification to keep out invaders of Asgard. After some debate, the gods agree to his conditions, but placed 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 was allowed.
To the surprise of the gods, Svaðilfari performs twice the deeds of strength as the builder, and hauls enormous rocks. This allows the Hrimthurs to make 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. In an unanimous agreement, along with most trouble, Loki was to blame. They then declared that he would deserve a horrible death if he could not find a scheme to make the builder forfeit his payment, and threatened to attack him.
In fear, Loki swore oaths that he would devise a scheme to make the Hrimthurs forfeit the payment, no matter what it would cause himself. That night, when the builder drove out to fetch stone with Svaðilfari, out of the woods ran out a mare. The mare, who is actually Loki in disguise, neighs at Svaðilfari, who becomes frantic, tears apart his tackle and runs towards the mare, realizing that it was in heat. The mare runs back into the wood with Svaðilfari following and the builder chases after. As the two horses run around all night, it caused the building work to be halted, and the previous momentum that the builder had been able to maintain could not be continued.
When the Aesir realize that the builder is a Hrimthurs, they disregard their previous oaths with him and call for Thor. Thor arrives and kills the builder by smashing his skull into shards with Mjölnir. However, Loki already had equine copulation with Svaðilfari, and therefore, somewhat later, Loki gave birth to a gray foal with eight legs, the horse Sleipnir, "the best horse among gods and men".