Brunnmigi are spirits who urinated in wells and are able to assume forms of their will. They mostly appear as a mixture of bear and wolf.
Magnus Chase and the Gods of Asgard[]
The Hammer of Thor[]
Inge and Hearth tell Magnus about the Brunnmigi named Pees-in-the-well (the only known one) who attacked and killed Hearthstone's brother Andiron in the form of a blue bear-wolf while the former's back was turned. Later, Alderman killed the Brunnmigi and made Hearth skin the creature all by himself. After Hearth skinned the creature, its hide was treated and made into a carpet which served as the base for Hearth's wergild payment for his brother's death.