Brunnmigi are spirits who urinated in wells and are able to assume forms of their will.
Magnus Chase and the Gods of Asgard
The Hammer of Thor
Inge tells 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.
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