The Norse god who is the mother of the eight-legged horse Sleipnir is Loki.
The Prose Edda tells that Loki shapeshifted into a mare to distract Svaðilfari, a powerful stallion helping an unnamed builder construct Asgard’s fortifications. The diversion prevented the builder from meeting his deadline, but Loki’s encounter with Svaðilfari had an unexpected result: Loki later gave birth to a gray foal with eight legs.
That foal was Sleipnir, described as the best of all horses. Odin eventually became Sleipnir’s rider, and the horse could travel through the air and across the sea. Sleipnir also carried the god Hermóðr on his journey to Hel after Baldr’s death.
The wording can seem surprising because Loki is usually presented as male. Norse myths, however, repeatedly portray Loki as a shapeshifter, and this story specifically makes Loki the birth parent. Svaðilfari is the stallion and biological father in the episode, but Loki is the figure who gives birth.