Sleipnir is an eight-legged stallion. He is the son of Loki (in the guise of a mare) and Svadilfari the stallion, but loyal to Odin.
Sleipnir was sometimes said to have six legs with a foot in all of the worlds, and sometimes said to have a normal amount of feet. Sleipnir was, however, usually considered to be very strong and be able to travel between the worlds fairly easily.