Bragla

 And if you don 't listen closely to your father and I, then a bragla will surely lure you into its trap. 

-Woman to her child, as a warning

Braglas are dangerous goblin-like creatures with the ability to shapeshift into a beautiful black mare. If any foolish enough to mount the mare, it will turn into a horrid creature, its skin rotting off with bones visibly to the eye. It will then descend into the body of water it calls its home, taking the person who mounted the horse with him. There it will change to its own true form, and the last thing the victim will see, is the unnaturally wide smile with rows of sharp teeth of the Bragla.