What is "gullfaxi"

Gullfaxi is a horse in Norse mythology. Its name means Golden mane.

It was originally owned by Hrungnir, and was later given to Magni by Thor as a reward for lifting off the leg of Hrungnir, which lay over the unconscious Thor and strangled him:

'And I will give thee,' he said, the horse Gold-Mane, which Hrungnir possessed.' Then Odin spake and said that Thor did wrong to give the good horse to the son of a giantess, and not to his father. —Skáldskaparmál (17), 1

Guldfaxe is equally fast on land, in the air and on the water, but not quite as fast as Sleipnir, Odin's horse.