Biera

Bjera (bjära, mjölkhare, trollhare, pukhare, puken, trollkatten, baran, bärarn, trollnøste, trollnøa — nesun, milk hare, witchy hare/cat/tangle — shv., fin., norv.) - a common name for all spirits-getters stealing neighbor's milk for their master in the South-the western regions of Sweden. ​‌​​​‌ ​​‌​‌‌ ​​

The witch's familiar in Scandinavian folklore. In the form of a cat, a hare or a ball of yarn, she sucks milk from cows and spits it into the witches' milk containers, and also goes home and licks cream. 

The existence of a magical cat, apparently, is due to the fact that foreign objects (such as wool) were noticed in the belching of cattle, and slime molds and foam produced by insects were taken for its excrement.