Kelpies are horse-like immortal water spirits.
Background[]
They are often malignant, who live in rivers and lakes. Telgrin set up one to guard a bridge across a river that lead to his floating castle.[1] Kelpies like apples.[2] Those living in rivers are a type of river spirit. Before the coming of castle, there hadn't been any kelpies in Daventry for years.[3]
A kelpie has a long head, very much like that of a horse, save that it is strangely translucent and shimmering, and its mane is the deep green of an aquatic plant. It has dark eyes that are wild and frightening.[4] It has a glistening sinewy neck. It's great body is physically solid, but is slippery and elusive. Its midriff is slick. Water sprays from its nostrils. It's muzzle is filled with many teeth, which it can display in a hideous sort of grin. It is able to move its hind quarters in a bucking motion.
They speak in a high, airy voice.
It is in the kelpie's nature to lurk under bridges, and to drown unwary souls who try to cross.
Behind the scenes[]
Beyond the obvious inspiration from mythology. The kelpie has a conversation about his nature to kill others after being forced to take Alexander across the river, which is very reminiscent to the fable "The Scorpion and the Frog".
Perhaps somewhat physically similar to the seahorse.