Loki with his son, 8 legged Sleipnir
Sorry for mixing Marvel universe with authentic legend, but they look so cute together xD
In short words:
Svašilfari 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 Svašilfari and make the build impossible to complete
And so... later, Loki gave birth to Sleipnir, "the best horse among gods and men", later owned by Odin.
blame those two: