Brunnmigi

Brunnmigi are monstrous spirits from Norse folklore and mythology often envisioned as an unnaturally large fox or hybrid-creature that is infamous for defiling wells, often by urinating in them and thus polluting the water.

In ancient times many villages and towns relied on wells for survival and thus the Brunnmigi could be a truly devastating monster as the polluted water would cause drought or sickness in humans and animals alike - thankfully they are a very rare creature and spoken of only in small numbers (however the term also came to be used as a word of foxes in general).