Karl

Karl is a näcken is a shapeshifting water sprites. He first appear in "A Fistful of Warlocks".

In the series
He is a huge, foul-smelling water dweller, capable of assuming the shape of a horse. Having lost a bet with Anastasia Luccio, he's bound to assist her in the chase of Heinrich Kemmler.

Trivia
Näcken is the swedish name for male water spirits, invariably male, who often entice women and children to drown in lakes or streams by playing the violin.