Kelpie

A kelpie was a creature that resembled a mass of seaweed. It could alter its form to appear as a woman, a hippocampus, or a green horse, then charm a person into thinking it was in trouble and needed rescuing. Upon the person attempting to rescue the person, the kelpie grappled the victim and held it in the water until it drowned.

History
In 1361 DR, the illithid Vestress used the human Dagmar to plant kelpies in the water in her attempt to conquer the island of Ruathym.

Novels

 * Tangled Webs

Sourcebooks

 * Sea of Fallen Stars
 * Volo's Guide to Cormyr