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.
Unlike other dogs Cù Sìth is around the size of a cow with it's size it makes Cù Sìth job easier to bring people to fairy mounds or to kill them and take their souls. Even though the canine took souls to the afterlife or would take women to help rise the children that fairies had. The Cù Sìth would give warnings of the the fate that would come if no one prepared things after seeing or hearing the Cù Sìth.
Despite that the Cù Sìth kidnaps women forcing them to help fairies rise their kids. Cù Sìth only does this because fairies aren't capable of supplying milk for their babies and can't rise them. While fairies can't produce milk and can't provide anything else to take care of their babies fairies will sometimes steal a human child and replace it with their owns.
Cù Sìth and the fairies steal women to put the responsibility on the fairies baby on the mortal human. It is also the same reason why fairies steal children and repacle them with their own child they put the responsibility on someone since they are incapable of taking care of their own child. Because they can't provide what their children need and because they can't live the way humans do fairies are envious and jealous of humans but mostly human females.
It makes fairies angry to see humans living happy lives and how they can rise their human kids with none of problems that fairies have. It also saddens them to know they can never experience that joy humans have with their kids. Even though it's wrong but fairies have to steal women or disguise their fairy child as a human child to make sure their babies get the proper care.
While fairies steal kids and force humans to rise their own babies fairies are not only incapable of producing, providing, rising and helping their children. Legends also mention a lot of fairies are also incapable to give birth and can't have kids. This is another reason why they steal human children. The fairies that can't give birth don't replace the stolen child with one of their own. However the fairies that can't give birth are also incapable of providing things for the human babies they kidnap similar to how the fairies who give birth can't provide for their fairy babies.
That is the reason why Cù Sìth helps the fairies because despite their many powers they can do anything to help their children. This is because in myths and legends fairies are described as being spirits and because spirits are dead or undead they aren't capable of being able to do things that humans or living beings are capable of.
Cù Sìth is a phantom, reaping, fairy dog who collects souls just like the Grim Reaper although while the Grim Reaper takes souls to rest. In Celtic lore the fairy realm is described as being an afterlife so instead of taking souls to rest like the Grim Reaper Cù Sìth takes the souls of women for them to provide use for the fairies.