Sleipnir

Thor's origin
Thor had eight steeds that he had to pull his chariot besides Toothgnasher and Toothgrinder.

Firegnaw, Slaughterbit, Swamptooth, Snow Harpy, Warhoof, Smokemare, Stormbringer, and Mudbrute. When all eight were eaten by mortals that had been tricked by Loki, Thor took their bones back to Asgard and using a dangerous form of magic, brought them back as one - Sleipnir.

Loki's origin
According to other accounts, Sleipnir is the child of Loki and the stallion Svaðilfari.