Nuckalavees are a type of Faerie creature.
A nukalavee appears to be a man with two swords, one in each hand, riding on a horse that seems impossibly nimble. However, both “man” and “horse” are the same slimy dark green color. The nukalavee can sprout new arms out of its body and wield weapons with them (usually swords or clubs).
As it moves, the horse leaves a trail of water behind it, and water gushes from its wounds as it fights.
Once slain, the nukalavee slowly melts away, leaving only a pile of disgusting green slime behind, looking much like the algae on a pond.