Svaðilfari was a stallion who was owned by a giant. He helped his owner build a wall around Asgard. It was he who sired Odin's horse, Sleipnir on Loki while the god was in the form of a mare in heat. By doing this Loki prevented Svaðilfari's owner, a giant, from completing the wall he was building around Asgard. Svaðilfari helped his master by hauling enormous rocks and performing twice the deeds of strength of his master. However, three days before summer (the agreed deadline), Loki lured away Svaðilfari, leaving the giant unable to complete his job.
Had the unnamed giant been able to do so, his agreed payment from the gods would have been the goddess Freyja, the sun and the moon. Freyja was not pleased by her part in the payment and threatened Loki (as did the other gods when it looked like the giant would succeed) if Loki did not somehow prevent the giant from completing the walls.