The Monster of Glamis, sometimes referred to as the Horror of Glamis, was allegedly a surviving deformed member of the Bowes-Lyon family, kept in seclusion in Glamis Castle, Scotland.
It is difficult to determine whether the account of the 'Monster' is factual or not. The story was discussed during the latter half of the nineteenth century, when "Miss M. Gilchrist, writing in 1885, was not only confident that such a monster did actually exist, but even described him - half frog, half man!", also claiming he was the rightful earl.  The earliest surviving reference dates from 1908, where it was claimed "...in the Castle of Glamis is a secret chamber. In this chamber is confined a monster, who is the rightful heir to the title and property, but who is so unpresentable that it is necessary to keep him out of sight and out of possession". 
The story of Thomas Lyon-Bowes' survival (as given by James Wentworth Day) appears to have started in local villages as the result of an account by the midwife (whose name was not recorded). The deformed child was alleged to have been in rude health when the midwife left, causing suspicion when his death was announced a day or two later. The child Thomas has no gravestone, a matter which tends to support the initial rumours. (Thomas had been baptised as a Christian on birth.) This part of the story of Thomas did not become current until the 1960s, when reputed family accounts were first published.
The Queen Mother's biographer, Michael Thornton visited Glamis and was told by the sixteenth Earl that the entrance to the chamber where Thomas lived had been bricked up after his death.
In the story Vengeance for a Lonely Man (London, Headline, 1992.) by Simon Green, the plot and some events seems to be inspired heavily by the tale of The Monster and to a lesser extent Glamis Castle itself.
The short story The Horror at Chilton Castle by Joseph Payne Brennan also has striking similarities to the tale of the Monster.
In the comic book series The Invisibles, the character of the Moonchild is said to be the Monster of Glamis.
in the novel "Death at Glamis Castle" by Robin Paige the Monster of Glamis tale is stirred up during a murder investigation.
in the novel Casanova's Chinese Restaurant by Anthony Powell (vol 5 of A Dance to the Music of Time), when the narrator Nick learns of his friend Stringham's seclusion due to alcoholism,"I felt a pang of horror at the way his family now talked of Stringham: as if he had been put away from view like a person suffering from a horrible, unmentionable disease, or become some terrifying, legendary figure, fearful as the Glamis monster, about whom it was appropriate to joke as dreadful to behold, but at the same time a being past serious credence."
In the 1940s, French surrealist author Maurice Sandoz wrote "The Maze," a novel clearly based on the legend of the Glamis monster. In this story, however, the "monster" is a kindly, long-lived Scottish nobleman who had the misfortune to be born in the shape of a toad (or frog). His existence is kept secret, and he manages his affairs and property through a series of nephews. In 1953, the novel was filmed by director William Cameron Menzies as The Maze.