Source: foxinshadow.deviantart.comLoki with his son, 8 legged Sleipnir /)^3^(\
Sorry for mixing Marvel universe with authentic legend, but they look so cute together xD
In short words:
Svaðilfari was a stallion, owned by ice giant that took up the task of building a wall around Asgard in less than a year. In return the giant was allowed to take goddess Freyja as a wife. Loki took the form of a mare, to seduce Svaðilfari and make the build impossible to complete :D And so… later, Loki gave birth to Sleipnir, “the best horse among gods and men”, later owned by Odin.blame Rae and Vagabond
EHEM, YEAH

