Once there was a wall destroyed in the midst of the Aesir and Vanir war. A stranger came forth willing to rebuild the Asgard wall.
Get back here Stallion!!!!
The stranger asked that he could use his stallion, and in return for building he would receive the sun, moon, and goddess Freya. Although other gods were hesitant, Loki approved the use of the stallion.
The stranger begins rebuilding the wall with the help of the stallion. The horse could haul many heavy stones with ease, the Gods soon became worried for they were in danger of losing the sun, moon, and Freya.
With the fear of losing 3 of the most important things and being killed by Thor, Loki had to send a whining mare to attract the stallion into the woods where the stranger could never catch him.
Without his magical stallion the stranger could not finish the Asgard wall. Therefore when Thor returned, he took one blow at the strangers head and shattered his skull into pieces.
A few months later, Loki gave birth to a colt that was identical to the stallion, except he had 8 legs and could fly through the air and gallop over the sea. The Gods called the colt Sleipnir, and from then on he was the greatest horse of all 9 worlds.