In Scottish folklore, the large green fairy hound that serves as an omen of death is the Cù Sìth.
The Cù Sìth is a mythical hound of the Scottish Highlands, said to roam remote moors and shelter in rocky clefts. Folklore describes it as roughly the size of a small cow, with a shaggy dark-green coat, yellowish fur near the feet, and a long tail that may be coiled or plaited.
Its most frightening feature is its supernatural call. The hound can hunt silently but is said to bark only three times, with the sound carrying for miles. Anyone who hears it must reach safety before the third bark; otherwise terror itself may cause death. The creature was also believed capable of taking the souls of living people.
The name is often confused with the Irish cú sídhe, an Irish Gaelic form of the same broader fairy-hound tradition. It also resembles the Welsh Cŵn Annwn and should not be confused with the Cat Sìth, a fairy cat. The Scottish spelling Cù Sìth is therefore the best answer for this question.