technically Slepneir is supposed to be an eight legged horse. The offspring of Loki (as a mare) and a giant stone builder's horse. (Stories pretty messed up the gods sent loki to delay the builder because if he got the job done in a specific amount of time he could have Freyja's hand in marriage, i believe it was her, and they didn't want it so loki changed into the mare distracted the horse and next thing he's pregnant with Odin's steed)
