The same paper gave an account of the creature's appearance. As "large as a stirk, it had the appearance of a deerhound except the tail was curved." While lacking the usual feature of fiery red eyes and remarkable blackness, the prodigious size of the Cu Glas puts it very much in the tradition of its canine cousins.
Sometimes, it is said, men walking home would encounter the shadowy beast in a glen for a few fleeting seconds, but knew that sad tidings would soon befall him.
Unusually, two tale exists to explain the presence of the spectral beast (black dogs rarely have an origin story). The tales differ slightly, but both involve the aforementioned McDonald clan.
In one, the patriarch of the McDonalds raised his faithful deerhound from a pup until he was forced to fight in some of the wars of the period and was away from home for several years. In his absence, the hound grew fierce and wild and had taken up residence with her pups on an island. Despite warnings of her ferocity, McDonald was sure she would remember her old master and rowed to the island, whereupon the dog leapt at his throat and killed him outright. The villagers destroyed the dog but her spirit still remained to foretell of woe to lonely travellers in the glens.
The second version of the story differs slightly in that it was one of the pups who killed unlucky McDonald, thinking that he had come to hurt its mother.
Whether McDonald was real or invented, the tale of his spectral hound still haunts the desolate hills around Morar. Author Alistair McLeod seems to have used the story as background for one of his stories in his 1986 compendium As Birds Bring Forth The Sun.