The Aesir did not have a wall around their home, Asgard, as it had been destroyed during the war with the other gods, the Vanir. There was still much trouble with giants and the Aesir were vulnerable to attack. As luck would have it a mason called Blast arrived and offered to build a wall. But his price was high, he wanted the sun and the moon and Freya for his wife.
Now the gods needed this wall but they couldn't meet the price asked by the mason. Then the shapeshifter Loki came up with a clever idéa. He suggested that they agree to pay if the wall was finnished within six months. The gods believed that it would be imposible to build the wall in six months and they agreed to do as Loki suggested. They figured that then they would at least have half a wall.
The mason agreed but insisted that he be allowed to use his stallion Svadilfari to help him work. The gods didn't se any harm in this and the mason got to work.
Not only did he build a fine wall, he built it quickly too! So quickly in fact that the gods got really worried. They thought that he might actally finish in time and then they would have to pay him. Odin especially was furious with Loki. He threatend to kill Loki if he didn't do something to stop the mason from finihing in time.
Loki, not wanting to die, got to work. He had noticed that it was the stallion that was doing all the heavy work he devised a plan to get rid of him. Loki turned into a mare and lured the stallion away into the forest. He made sure that Svadilfari was away from the builder for a day and a night. When the stallion returned to his master it was too late to finish the wall.
Blast was in a terrible rage over his failiure to complete the wall. Whilst angry he accidentally revealed himself to be a rock giant. Thor wasn't having a giant anywhere near his home and he killed Blast with his hammer.
Loki didn't turn up untill nine months later and when he did he was leading a colt with eight legs. He presented it to Odin and told him that it's name was Sleipner. It grew into a fine horse that , because of its eight legs, could travel through water, air and over land
twice as fast as any other horse.