Kelpies look like black ponies. It will drown anyone who tries to ride it and eat them.  It looks like a lost pony that walks on shorelines, but upon closer inspection it is dripping wet with skin like a seal.  Kelpies wail before storms and cause waters to rise and flood.  If you can get a bridle it, then it must do your bidding, but good luck if it should wiggled out of the bridle.
* photo courtesy of Arthur Spiderwick's Field Guide