The earliest account is George Waldron's, in his Description of the Isle of Man printed in 1744 from his residence on the Island in the 1720s. They said, he wrote, that an apparition called in their language the Mauthe Doog, in the shape of a large black spaniel with curled shaggy hair, used to haunt Peel Castle, and had been frequently seen in every room, but particularly in the guard chamber, where as soon as candles were lighted it came and lay down before the fire in the presence of all the soldiers. By long custom they lost great part of their terror, but they still believed it an evil spirit which only waited permission to do them hurt, and for that reason forbore swearing and all profane discourse in its company. None cared to be left alone with it. Since one soldier had to lock the castle gates at a certain hour and carry the keys to the Captain, whose apartment was reached by a passage through a church, they agreed that whoever was to take the keys should be accompanied by the man who went the night before, for the dog was always seen to come out of that passage at the close of day and go back to it at dawn, which made them look on the place as its peculiar residence.

One night, Waldron went on, a soldier who was drunk laughed at the simplicity of his companions and, though it was not his turn, snatched up the keys and went out of the guard room, swearing that he desired nothing more than that the Mauthe Doog should follow him, for he would try whether it were dog or devil. Some time after his departure a great noise was heard, but nobody had the boldness to see what occasioned it. When he returned he was sober and silent enough, for he was never heard to speak again, and though for the three days he lived he was entreated to speak or make some sign of what had happened to him, nothing intelligible could be got from him, and by the distortion of his limbs and features it was guessed that he died in agonies greater than is common in a natural death. The Mauthe Doog was never seen in the castle again, nor would anyone go through that passage, which was closed up and another way made. Waldron dated the accident about threescore years before his own time, which places it in the 1660s, and said he had it attested by several, especially by an old soldier who assured him he had seen the dog oftener than he had hairs on his head.

Everything since has followed Waldron. George Woods copied the story into his account of Peel Castle in 1811 and noted that Walter Scott had drawn on it in the Lay of the Last Minstrel, where a knight lies speechless like him of whom the story ran that spoke the spectre-hound in Man. Jenkinson's guidebook pointed visitors to the guard room where the Moddey Dhoo used to come and sit silently among the soldiers, and Moore printed Waldron's text in 1891 under the heading Legend of the Black Dog, Moddey Doo, describing the Moddey Dhoo in his introduction as one of the Island's spirits or fiends of evil and malicious nature, beside the Buggane. Sophia Morrison retold it for children in 1911 with the date fixed in the days of Charles II and Charles, Earl of Derby. By then the name had become a common noun. W. Walter Gill, in 1929, used moddey dhoo for any haunting black dog: one at Balladoole in company with a White Lady, one on the road near Ballamodda, a headless one in the lane at Ballagilbert like that at Hango, and a black dog reported in 1852 that howled and screamed and chased people round Castletown as far as the gallows post at Hango but dared not go beyond, and he noted that the word buggane in Manx use covered almost all apparitions of the gloomier sort, black dogs in particular.