Kelpie

A shapeshifting water spirit of Scottish lore, they are tricksters that lure weary travelers to ride it, then drag their victims to their deaths with their superglue-like skin and drown them. They’ll then devour their victim, all except for their liver. The kelpie is often described to take on the form of a horse, but has also been known to take on the form of a human to further lure its prey in. In some stories. The human form, in some accounts, is said to retain its hooves and its true self is betrayed by water reeds stuck in its hair. Its equine form is often described as either pure white, pure black, or a dark coat with a greenish tint to it. Its skin is akin to that of a seal’s, slick and smooth, and has properties similar to superglue.

They can be captured, it is an ill omen to do so. As creatures sometimes associated with the fae and the faerie courts, they are incredibly vindictive and can take their capture as a grievous slight. Capture can include the use of iron, as faeries are weak to it. An iron horse bit and reins, alongside a horse saddle imprinted with a Celtic knot cross, can ensure it will follow the orders of its captor. However, due to their spiteful nature, if they are ever freed, they will turn on the one who seized them. If their original captor is dead, they will turn on their next of kin.

There are ways to escape a kelpie, though. Like all fae, they enjoy stories and can be used to barter one's way out with one. Take caution if this course is taken: they're tricksters and have gained the title for a good reason. They will try to find a loophole to exploit in order to get both a meal and a story out of an encounter.

It is said these trickster spirits can be killed, like a werewolf, with silver. However, it is preferable to use iron, as it is a more common weakness to faerie-lore creatures.

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An submission for the Teen Wolf monster design contest. I don't doubt I'll probably lose, but hey, it was fun either way, because everyone needs more horse monsters to love.