Gullinbursti is a boar

Boar

Wild boar, also wild pig, is a species of the pig genus Sus, part of the biological family Suidae. The species includes many subspecies. It is the wild ancestor of the domestic pig, an animal with which it freely hybridises...

 in Norse mythology

Norse mythology

Norse mythology, a subset of Germanic mythology, is the overall term for the myths, legends and beliefs about supernatural beings of Norse pagans. It flourished prior to the Christianization of Scandinavia, during the Early Middle Ages, and passed into Nordic folklore, with some aspects surviving...

.

When Loki

Loki

In Norse mythology, Loki or Loke is a god or jötunn . Loki is the son of Fárbauti and Laufey, and the brother of Helblindi and Býleistr. By the jötunn Angrboða, Loki is the father of Hel, the wolf Fenrir, and the world serpent Jörmungandr. By his wife Sigyn, Loki is the father of Nari or Narfi...

 had Sif

Sif

In Norse mythology, Sif is a goddess associated with earth. Sif is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson, and in the poetry of skalds...

's hair, Freyr

Freyr

Freyr is one of the most important gods of Norse paganism. Freyr was highly associated with farming, weather and, as a phallic fertility god, Freyr "bestows peace and pleasure on mortals"...

's ship Skíðblaðnir

Skíðblaðnir

In Norse mythology, Skíðblaðnir is the best of ships. Skíðblaðnir is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and in the Prose Edda and Heimskringla, both written in the 13th century by Snorri Sturluson...

 and Odin

Odin

Odin is a major god in Norse mythology and the ruler of Asgard. Homologous with the Anglo-Saxon "Wōden" and the Old High German "Wotan", the name is descended from Proto-Germanic "*Wodanaz" or "*Wōđanaz"....

's spear Gungnir

Gungnir

In Norse mythology, Gungnir is the spear of the god Odin.-Poetic Edda:In the Poetic Edda poem Völuspá, the Æsir-Vanir War is described as officially starting when Odin throws a spear over the heads of an assembly of Vanir gods. Whether or not this was specifically Gungnir is, however, unstated...

 fashioned by the Sons of Ivaldi

Sons of Ivaldi

In Norse mythology, the Sons of Ivaldi are a group of dwarfs who fashion Skidbladnir, the ship of Freyr, and the Gungnir, the spear of Odin, as well as golden hair for Sif to replace what Loki had cut off....

, he bet his own head with Brokkr that his brother Eitri

Eitri

In Norse mythology, Eitri is a dwarf and the brother of Brokkr.According to Skáldskaparmál, when Loki had Sif's hair, Freyr's ship Skidbladnir and Odin's spear Gungnir fashioned by the Sons of Ivaldi, he bet his own head with Brokkr that Eitri would not have been able to make items that matched...

 (Sindri

Sindri (Norse mythology)

In Norse mythology, Sindri is the name of both a character and a hall that will serve as a dwelling place for the souls of the virtuous after Ragnarök.-A dwarf?:...

) wouldn't 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.

The story of Gullinbursti's creation is related in the Skáldskaparmál

Skáldskaparmál

The second part of Snorri Sturluson's Prose Edda the Skáldskaparmál or "language of poetry" is effectively a dialogue between the Norse god of the sea, Ægir and Bragi, the god of poetry, in which both Norse mythology and discourse on the nature of poetry are intertwined...

 section of Snorri Sturluson

Snorri Sturluson

Snorri Sturluson was an Icelandic historian, poet, and politician. He was twice elected lawspeaker at the Icelandic parliament, the Althing...

's Prose Edda

Prose Edda

The Prose Edda, also known as the Younger Edda, Snorri's Edda or simply Edda, is an Icelandic collection of four sections interspersed with excerpts from earlier skaldic and Eddic poetry containing tales from Nordic mythology...

.

[Þ]á lagði Sindri svínskinn í aflinn ok bað blása Brokk ok létta eigi fyrr en hann tæki þat ór aflinum, er hann hafði í lagt. En þegar er hann var genginn ór smiðjunni, en hinn blés, þá settist fluga ein á hönd honum ok kroppaði, en hann blés sem áðr, þar til er smiðrinn tók ór aflinum, ok var þat göltr, ok var burstin ór gulli. ... 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. - Skáldskaparmál ch. 44

"Sindri laid a pigskin in the hearth and bade Brokkr blow, and did not cease work until he took out of the hearth that which he had laid therein. 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." - Translation by Arthur Gilchrist Brodeur

According to Húsdrápa

Húsdrápa

Húsdrápa is a skaldic poem partially preserved in the Prose Edda where disjoint stanzas of it are quoted. It is attributed to the skald Úlfr Uggason. The poem describes mythological scenes carved on kitchen panels...

, Freyr rode Gullinbursti to Baldr's funeral, while in Gylfaginning

Gylfaginning

Gylfaginning, or the Tricking of Gylfi , is the first part of Snorri Sturluson's Prose Edda after Prologue. The Gylfaginning deals with the creation and destruction of the world of the Norse gods, and many other aspects of Norse mythology...

, Snorri states that Freyr rode to the funeral in a chariot pulled by the boar.

The boar is also known as Slíðrugtanni (sometimes Anglicized to "Slidrugtanni").