Fuath

A Fuath (plural. Fuathan) is Scottish Gaelic for 'hatred', and is primarily from the Scottish Highlands. They are malevolent spirits, primarily related to water. They reside in streams & rivers, lochs, and the sea. There are intermittent stories of intermarriage of them with humans, especially the females, and the children have a mane and a tail. Besides meaning 'hatred', it can also mean 'aversion', and comes from the old Irish word fúath meaning 'hate, likeness'. It is regularly tied to ghosts or spirits, and alternate names for them are: 'arrachd' or 'fuath-arrachd'. A number of spirits fall under this title including the Brownie & related creatures, as well as the snake / dragon creature known as a 'Berithir'.
They seem to fall in between the Undead and the Fey, and are linked to both. Most speak either Gaelic and/or Brythonic, and a few also seem to speak Sylvan. It is likely some of the more malevolent are from the Shadowfell, and have passed through to the Feywild. Others have gone the other way, explaining why only some speak Sylvan, while others appear or are undead in nature. Both the Raven Queen & the Unseelie Court seems to have contact with them the most, while sections of the Fuath, such as the Brownies, have contact with the Seelie Court.