The Cu Sith or Black Angus is a Faery Dog. In Wales they are the Cwn Annwn, dog of the Otherworld, and are seen crossing the moors and wastelands at night. Likely this lore was inspiration for Arthur Conan Doyle's 'HOUND OF THE BASKERVILLES'.
It is a large black dog with gleaming yellow eyes, and often is seen standing and walking as a man. He comes to show himself to those who will die. Scots Lowlanders often describe him as having horns like the devil, but this is a much later Christian interpretation of the lore.
When he presents himself to his target, he is often seen by others as well. The last execution of a witch in the Britain was a man killed in 1945 by villagers by means of 'Hayhook and Billfork'. A billfork is a pitchfork and it is driven through the victim's neck, pinning him to the ground. The old usage of 'iron' to discharge a witch's powers. Then the Hayhook (particularly nasty blade, like a huge knife, created to grab hold of haybales for loading) is used to carve a sign of the cross into his chest. The old TV show "Thiller" hosted by the great Boris Karloff in the late 1958-59, had an episode based on this true incident entitled 'HAYHOOK and BILLFORK', and though many of the facts were corrupted, the basic tale remained. They also made mention of the Black Dog. The real Scotland yard official investigating the murder saw it, as well as many of the villagers, and he recorded it his report to Scotland Yard. No villager had a black dog and there were no strays in the area. They had a habit of killing any wandering black dog for fear of it being the Cu Sithe.
My own mother saw the Cu Sithe when she was child. And before you immediately say she was an highly imaginative child, her foster-father, foster-sister(17) and two foster-brothers(16, 15) witnessed the Black dog, walking upright like a man and coming for my mother. Also, due to the death of my grandfather, she was raised in the States from age 2 to 9 and never even heard the tales of Cu Sithe until after the incident. When she told her adventure, only then did someone tell her about the Angus Dubh. Someday, I will be add her story, a story she told many times over during her life, a story she recorded for me though she was dying of cancer. I promised her to tell it...and I shall.