Brewer's: Mauthe Dog
A “spectre hound” that for many years haunted the ancient castle of Peel town, in the Isle of Man. This black spaniel used to enter the guard-room as soon as candles were lighted, and leave it at day-break. While this spectre-dog was present the soldiers forebore all oaths and profane talk. One day a drunken trooper entered the guard-house alone out of bravado, but lost his speech and died in three days. Scott refers to it in his Lay of the Last Minstrel, vi. stanza 26.
For the legend, see a long note at the beginning of Scott's Peveril of the Peak, chapter xv.