Wikipedia
The nuckelavee or nuckalavee is a horse-like demon from Orcadian mythology that combines equine and human elements. It has its origins in Norse mythology, and is the most horrible of all the demons of the Scottish islands. The nuck component of its name may be cognate with Nick in Old Nick, a name for the Christian Devil. The nuckelavee's breath was thought to wilt crops and sicken livestock, and the creature was held responsible for droughts and epidemics on land despite its being predominantly a sea-dweller.
A graphic description of the nuckelavee as it appears on land was given by an islander who claimed to have had a confrontation with it, but accounts describing the details of the creature's appearance are inconsistent. In common with many other sea monsters it is unable to tolerate fresh water, therefore those it is pursuing have only to cross a river or stream to be rid of it. The nuckelavee is kept in confinement during the summer months by the Mither o' the Sea, an ancient Orcadian spirit, and the only one able to control it.
Orcadian folklore had a strong Scandinavian influence, and it may be that the nuckelavee is a composite of a water horse from Celtic mythology and a creature imported by the Norsemen. As with similar malevolent entities such as the kelpie, it possibly offered an explanation for incidents that islanders in ancient times could not otherwise understand.