Kelpie
This being from Scottish tradition is also known as the “water horse” and is linked to various myths about sea horses. It is an evil spirit allied with the devil and a great enemy of men. It tends to take on different appearances depending on the circumstances, though the most common is that of a beautiful horse, with which it deceives its victims.
The Kelpie allows itself to be mounted docilely, and once the rider is upon it, it drags them into the water and devours them.