He looks like a magnificent black horse with a starry hide, and wears medieval-style barding. He has the ability to fly. Sleipnir can change the number of his legs at will and sometimes has eight legs, although he did not take this form in his encounter with the Avalon World Tour travellers. 
Note: This character does not speak, therefore he does not have any lines.
- "Eye of the Storm" (First Appearance)
Real World Background
Sleipnir was Odin's horse in Norse mythology, and was particularly noted for having eight legs, although he is described in the legends as grey rather than black. (Some scholars of Norse mythology believe that this feature of his was thanks to Odin's status as a death-god. According to this theory, Sleipnir is a personification, of a sort, of a coffin, which is carried by four pallbearers, and thus can be viewed as having eight legs). According to the myths, he was born in this wise. A frost giant offered to build a mighty stone wall around Asgard, on the condition that, if he completed it before the end of winter, Odin give him in payment the sun and moon, and also Freya, the Norse goddess of love and beauty, for his wife. Odin disliked the demanded price, but, after Loki the trickster-god convinced him that the giant could not possibly complete the wall in that amount of time, agreed to it. What he and the other gods had not reckoned with was that the giant had a powerful work-horse, a stallion named Svaldifari, who hauled massive rocks for the wall to the building site, allowing the giant to build the wall with amazing swiftness.
The gods, alarmed at this development, blamed Loki for this state of affairs, and ordered him to do something about the problem. Loki did just that by shape-shifting into a mare and luring Svaldifari away from the building-site, thus preventing the giant from completing the wall by the deadline. However, Svaldifari managed to catch up with him, and Loki later on gave birth to Sleipnir.
Behind the Scenes
Slepnir was originally going to have eight legs in his appearance on Gargoyles, but it was determined that animating an eight legged horse would be too difficult for the overseas animation studio. The in-continuity explanation is that Slepnir can alter his form at will, including his number of legs, though it's never mentioned in the episode itself.
- Sleipnir at Wikipedia, the Free Encyclopedia