Neck

Neck/Nixie/Nix/Nixe/Nyx are shapeshifting water spirits who usually appear in human form.
==True shape of the Neck==
Although in recent times such creatures have usually been depicted as human in shape (albeit in many cases shapeshifting), the English Knucker is generally depicted as a wyrm or dragon, thus attesting to the survival of the other usage as any "water-being" rather than an exclusively humanoid creature.