Kelpie

The kelpie is a shapeshifting creature that lives in rivers and lakes. They typically choose either the form of a human female, or that of a horse with odd or aquatic features. If a person grabs onto the creature or tries to ride it's horse form, they will be bound to it, and the kelpie, can then proceed to throw itself into the river in order to drown the unfortunate soul.

Source
The kelpie originates from Scottish folklore.

Media
The Kelpie appear in the following media:

Novels

 * Berserk