Short answers, pulled from the story.
Sleipnir is an eight-legged grey horse ridden by the god Odin in Norse mythology. Its name comes from Old Norse words meaning "slippy" or "the slipper," and the sources describe it as the best of all horses among gods and men. Sleipnir is the child of Loki and the stallion Svaðilfari.
Sleipnir was born after Loki transformed into a mare to lure the stallion Svaðilfari away from an unnamed builder who was constructing a fortification for the gods. The distraction caused the builder to miss his deadline. Loki later gave birth to a grey foal with eight legs, which became Sleipnir.
Sleipnir appears in the Poetic Edda and the Prose Edda, both compiled or written in the 13th century, as well as in the 13th-century sagas Hervarar saga ok Heiðreks and Völsunga saga. Saxo Grammaticus's 12th-century Gesta Danorum is also considered by many scholars to contain an episode involving Sleipnir.
Scholars have proposed several interpretations. Hilda Ellis Davidson connects the eight legs to shamanic traditions, noting that eight-legged horses appear in the folklore of Siberian shamans, and suggests the image may also derive from a funeral bier carried by four pallbearers, giving a dead man a horse with eight legs. The Encyclopedia of Indo-European Culture theorizes the legs may be remnants of horse-associated divine twins found across Indo-European cultures.
The Tjängvide image stone and the Ardre VIII image stone, both 8th-century picture stones from the island of Gotland in Sweden, are generally accepted by scholars as depicting Sleipnir. Each stone shows a rider atop a horse with eight legs. The scene on the Tjängvide stone has been interpreted as a rider arriving at the world of the dead.
In chapter 13 of Völsunga saga, the hero Sigurðr encounters an old man who identifies a young grey horse as being from Sleipnir's kin and predicts it will become better than any other horse. Sigurðr names the horse Grani, and the old man is later revealed to be Odin.