Wikipedia
A fuath (plural fuathan; ) or vough (phonetic transcription), literally meaning "hate" in Scottish Gaelic, designates a class of malevolent Highland Gaelic mythological water spirits, inhabiting the sea, rivers, fresh water, or sea lochs.
As a generic term the fuath can include the beithir ( behir), peallaidh and ùruisg. The "fuath" in collected Gaelic folklore is substituted by the term " kelpie" in the English translation and commentary portions of John Francis Campbell's Popular Tales of the West Highlands. More broadly, this name is even given to highland or nature spirits.
Their appearance ranges from covered in shaggy, yellow fur to just having a mane down its back, webbed toes, tails with spikes, and no nose. They are prone to wearing green, whether it be a dress, robe, or kirtle, as it is the colour of faeries.
They sometimes intermarry with human beings (typically the female), whose offspring will share a mane, tail, and/or webbed digits. Their banes include sunlight and cold steel, which will kill them instantly. They grow restless upon crossing a stream.
An alternative name for this class of monsters is Arrachd.
Similarity or equivalence to the bean nighe or Northern Ireland's uisges have been noted.