Loki and the Giant
During the war between the Aesir and the Vanir, Asgard had lost its walls. One day, a man came on horseback and offered to build a wall stronger than the previous one. However, the price for his work was the sun, the moon, and the goddess Freyja as his wife. On the advice of the cunning god Loki, the gods agreed, but only on the condition that the work be completed in six months (something they naturally considered impossible).
Despite this, the man and his horse, Svadilfari, worked with such speed that the work was finished three days before the deadline. The gods were terrified, and so Loki, who could transform himself, took the form of a mare and managed to lure Svadilfari away so that the man would be unable to finish the wall in time.
The man then grew so angry that he began to grow and grow, revealing that he was in fact a rock giant, a lineage that hated the gods. Thor then slew him with a blow of his hammer.
Months later, Loki returned with a strange foal, sired by Loki and Svadilfari. This was Sleipnir, the future eight-legged steed of Odin, who ran faster than anything in the world and could carry his rider all the way to Hel, the land of the dead.