Loki

Yay! Kinsara put a drawing up ^.^ I'm very proud of it.
THIS IS NOT A CENTAUR.
It's Loki, the Norse god of chaos and mischief, turning into a mare
For the full story, see here: [link] (look up Sleipnir)
Basically, Asgard (the place where the gods live) was unprotected and a guy said he would build a wall if they would give him Freya as a wife, and also for the sun and moon. The gods said no way, but Loki told them he would be able to get them the wall without paying the price, and told them to tell him he had to build the wall in six months. They did so, and Loki transformed himself into a mare and lured the man's horse away, so the man wasn't able to finish the entire wall.
The result-- Loki returned with a colt named Sleipnir, who had eight legs and became Odin's horse.