Who better to explain these mysterious water horse shape-shifters than Rez, the kelpie stallion in "Dark Water"?
“The tales of kelpies were told to keep children away from the water, morality tales about trespass and trusting strangers.
A horse of incredible beauty would emerge from the water, or from an area near the water. Remember in those days, horses weren’t just
pets or companions. They were status, transportation, help in the fields. A good horse could elevate a family’s fortunes, so the mysterious horse was a powerful lure.
Usually, they were white but not always. In some stories, they have coats that always drip water. In others, their manes and tails are seaweed or their hooves are turned the opposite direction compared to land horses.
“The point was that the horse was supposed to lure the unsuspecting person onto his back or into the water. If the children, or men, or women touched the skin of the kelpie, their hands sank into the creature like tar. They could pull and struggle, but would end up bound tighter to the kelpie, which would gallop into the sea and drown them.”
--Excerpt from "Dark Water"