A piece three years in the making. I think it paid off, this is possible my greatest piece to date.

The kelpie is a water creature from Scottish legend. It is a shape shifter that can take on many forms to attract humans to it, from a beautiful man/woman to attract a lustful youth, to a group of children playing dangerously in order to alert a concerned adult. It's most commonly depicted as a beautiful white horse that lures in children who may want to pet or even ride it.
Whatever form it takes, once the unsuspecting human makes contact with it, it's beautiful skin will become blackened and sticky. So sticky that it will be impossible to let go of it. At this point, the kelpie will casually walk into the lake/river it was next to and drag its victim under the water to drown them and, presumably, eat them.

From what I've heard, there's been one survivor. A young boy whose friends all climbed the kelpie whereas he cautiously petted the creature with a single finger. Naturally, his friends couldn't dismount and were dragged into the water, never to be seen again. The boy only escaped because he had a knife which he used to cut off his finger.

I've heard of ways to tell if the handsome man/beautiful woman/majestic horse you're looking at is a kelpie. Apparently no matter what form they take, a kelpie will likely have seaweed in its hair/mane and its feet will be pointing backwards. These however seem to be later additions to the story and are likely nonsense.