Óðinn's eight-legged horse and one of Loki's three monstrous children. Capable of crossing any form of landscape and even across realms e.g. into the realm of the dead. Occasioanlly loaned to other members of the pantheon/mortal heroes.
Sleipnir is Born (Gylfaginning) The Death of Baldr (Gylfaginning) The Death of Jǫrundr