Ketta origins are from the Norse the Ketta are cat spirits Ketta means she-cat of a female cat but the word is also used to identify curtain fierce female spirits. The most famous Ketta is the being that Grendel the monster in Beowulf call mom. Some perceive a resemblance between Grendel mother and Black Annis (black Annis). The form the Ketta is a large cat, a woman or something in between. The defining characteristic seems to be extremely long sharp cat claws even if the rest of the body is human.
Judika Illes, Encyclopaedia of Spirits the ultimate guide to the magic of fairies, genies, demons, ghosts, gods and goddesses, 2009, Ketta, p 564