The Prose Edda tells Gullinbursti's origin as part of a single long episode in which Loki, having cut off Sif's hair as a prank, is forced to make amends by commissioning replacement treasures from the dwarves, and rashly bets his own head against the sons of Ivaldi that no one can outdo the gifts they produce. He turns to the brothers Brokkr and Eitri, who accept the challenge, and Eitri sets a pig's skin in the forge fire and tells Brokkr to work the bellows without stopping no matter what happens, or the working will fail.
Loki, aware he stands to lose the bet and his head with it, transforms into a horsefly and bites Brokkr repeatedly on the hand to break his rhythm, but Brokkr keeps pumping through the pain, and the skin emerges from the fire as Gullinbursti, a boar whose bristles shine gold bright enough to light the darkest road. The same forging session, without pause between pieces, also produces Draupnir, the ring that drips eight equal rings every ninth night, and Mjolnir, Thor's hammer, whose handle comes out shorter than intended because Loki's fly bit Brokkr on the eyelid at the crucial moment, the one flaw in an otherwise unbroken run of treasures.