Loki, in the shape of a mare, gave birth to Sleipnir after his union with the stallion Svadilfari. Sleipnir is the finest horse among gods and men and has eight legs.
Um várit eftir fœddi Loki fyl. Þat var grátt ok hafði átta fœtr ok er sá hestr beztr meðal goða ok manna. Þat heitir Sleipnir ok á hann Óðinn. Engi hestur getur hlaupið jafnhratt sem Sleipnir, hann gengr bæði á lofti ok á sjó.
The following spring Loki gave birth to a foal. It was grey and had eight legs and is the finest horse among gods and men. It is called Sleipnir and belongs to Odin. No horse can run as swiftly as Sleipnir; he can travel both through the air and across the sea.
Eddic quotations
Sleipnir er beztr meðal hesta ok Bifröst brúa.
Sleipnir is the finest of horses and Bifrost the best of bridges.