Nuckelavee

When pollution despoils a natural waterway, it draws the dreaded nuckelavee to it from the First World. This spirit of wrath is a grisly sight to behold: a horselike monstrosity with the gnarled upper body of a humanoid growing directly from its back. Further enhancing its awfulness, not a patch of skin exists on the misshapen hybrid form, as though it survived its own flaying.

When a nuckelavee rides forth from its domain, it wreaks a trail of destruction across the land surrounding its path. Nuckelavees are considered among the cruelest and most monstrous fey, seen by some as just desserts visited upon those who would befoul the waters of their homes. A nuckelavee, however, doesn't discriminate between those who pollute and those who merely have the misfortune to be in the wrong place at the wrong time.

Despite their vile reputation among humanoids, nuckelavees are generally respectful of their fey counterparts. Once pollution has been cleansed and water fey like naiads return to a body of water, nuckelavees will peacefully withdraw.