The kelpie is a shape-shifting water

horse that is believed to haunt the rivers and lochs of Scotland and Ireland.  The kelpie would lure people onto its back and dive into a loch to mysteriously vanish.  A kelpie's bridle could perform magic if stolen, and a human bridle on a kelpie could compel it to work for a human.  A similar creature in Orkney was called the Nuggle, and in Shetland, it is called the Shoopiltee.