Gullinbursti

When Loki had Sif's hair, Freyr's ship Skíðblaðnir, and Odin's spear Gungnir fashioned by the Sons of Ivaldi, he bet his own head with Brokkr that his brother Eitri (Sindri) would not have been able to make items to match the quality of those mentioned above.

So to make gifts to Freyr, Eitri threw a pig's skin into a furnace as Brokkr worked on the bellows, and together they manufactured the boar Gullinbursti which had bristles in its mane that glowed in the dark.

... Þá bar fram Brokkr sína gripi ... En Frey gaf hann göltinn ok sagði, at hann mátti renna loft ok lög nótt ok dag meira en hverr hestr ok aldri varð svá myrkt af nótt eða í myrkheimum, at eigi væri ærit ljós, þar er hann fór; svá lýsti af burstinni.

But when he went out of the smithy, while the other dwarf was blowing, straightway a fly settled upon his hand and stung: yet he blew on as before, until the smith took the work out of the hearth; and it was a boar, with mane and bristles of gold.

... Then Brokkr brought forward his gifts: ... to Freyr he gave the boar, saying that it could run through air and water better than any horse, and it could never become so dark with night or gloom of the Murky Regions that there should not be sufficient light where he went, such was the glow from its mane and bristles.