The Cù Sìth of Scottish mythology is an enormous, otherworldly hound, said to haunt the Scottish Highlands. Roughly the size of a cow or large calf, the Cù Sìth was said to be dark green in color with shaggy fur and a long braided or curled tail. In Irish mythology, the Cù Sìth was said to be an immense, coal-black hound with glowing or flaming eyes. 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).
According to legend, the creature was capable of hunting silently, but would occasionally let out three terrifying barks that could be heard for long distances, including by ships at sea. This was said to be a warning to farmers to lock up their women, lest the beast abduct them and take them to a fairy mound to supply milk for the children of the fairies.
Cù Sìth literally means "barrow hound". Supernatural dogs in Celtic myths are usually completely black, or white with red ears. The Cù Sìth's coloration is therefore highly unusual, although it may be derived from the green color often worn by Celtic fairies. When men heard its howl, they would lock up their women.