The Cù-Sith (Scottish Gaelic pronunciation: [kuː ʃiː]), plural Coin-Sìth (Scottish Gaelic pronunciation: [kɔːn ʃiː]) is a mythological hound found in Scotland and the Hebrides. A similar creature exists in Irish folklore (spelled Cú Sídhe), and it also bears some resemblance to the Welsh Cŵn Annwn.
According to Scottish folklore, the Cù-Sìth is said to be the size of a young bull with the appearance of a wolf. Its fur is shaggy, and usually cited as being dark green though sometimes white. Its tail is described as being long and either coiled up or plaited (braided). Its paws are described as being the width of a man's hand.
The Cù-Sìth is thought to make its home in the clefts of rocks in the Highlands, and also to roam the moors and highlands.
The Cù-Sìth was feared as a harbinger of death and would appear to bear away the soul of a person to the afterlife, similar to the manner of the Grim Reaper. In this role the Cù-Sìth holds in Scottish folklore a function similar to that of the Bean Sidhe, or banshee, in Irish folklore.
According to legend, the creature was capable of hunting silently, but would occasionally let out three terrifying bays, and only three, that could be heard for miles by those listening for it, even far out at sea. Those who hear the baying of the Cù-Sìth must reach safety by the third bark or be overcome with terror to the point of death.
It was also said the baying was a warning to lock up nursing women lest the beast abduct them and take them to a fairy mound (Scottish Gaelic: sìthean, pl. sìtheanan) to supply milk for the Sith (daoine sìth).