Heiðrún is a goat in Norse mythology, which consumes the foliage of the tree Læraðr and produces mead for the einherjar. She is described in the Poetic Edda and Prose Edda.
Geit sú er Heiðrún heitir stendr uppi á Valhöll ok bítr barr af limum trés þess er mjök er nafnfrægt, er Léraðr heitir, en ór spenum hennar rennr mjöðr sá er hon fyllir skapker hvern dag. Þat er svá mikit at allir einherjar verða fulldruknir af. -
A goat called Heiðrún stands up [on its hind-legs] in Valhalla biting the buds off the branches of that very famous tree which is called Lærað. From her teats runs the mead with which every day she fills a cauldron, which is so big that all the Einherjar can drink their fill from it. - Young's translation
In the Poetic Edda Heiðrún is mentioned twice. She is described in the Grímnismál in a way similar to Snorri's description.
Since Snorri quotes other strophes of Grímnismál it seems reasonable to assume that he knew this strophe too and used it as his source for his description of Heiðrún.
In the Hyndluljóð the giantess Hyndla (lit. bitch/she-dog) used the term "Heiðrún" to insult the goddess Freyja. Thorpe and some other translators translated the name straight to "she-goat".
Heiðrún's name is sometimes anglicized Heidrun, Heidhrun, Heithrun, Heidrún, Heithrún or Heidhrún.