A troll cat (also milk rabbit, troll ball) is the familiar of a witch in Scandinavian folklore. Either in the shape of a cat or a ball, troll cats sucked milk from cows and spat it out in the witches’ milk pails, and went into homes to lick up cream. Related creatures are milk hares and milk rabbits.
Source: Troll cat – Wikipedia
Tim B.
https://en.wikipedia.org/wiki/Tilberi