The peerage records Lord Glamis's first son as born and dead on 21 October 1821; a legend says he lived, hidden in the castle, and each heir was told the secret

The Monster of Glamis is a legend. The record is one line: the Scots Peerage lists the first child of Lord Glamis, after his 1820 marriage, as "a son, born and died 21 October 1821." The legend says the boy, the rightful heir, was born deformed and was hidden in a secret chamber, and that each heir was told the secret around his 21st birthday. The historian Mike Dash traced the story. Walter Scott visited Glamis in 1790 and wrote of a secret room but no occupant. Reports of a captive appear from the 1840s, and American newspapers printed the story in 1882 and 1904. The 13th Earl, Claude, told his wife, "I have heard the secret, and if you wish to please me you will never mention the subject again." In the 1960s the 16th Earl said he knew nothing and that the secret "may have died with my father." Brian Dunning notes that nearly all modern accounts trace back to James Wentworth Day's 1967 book, and calls the monster legend. Dash concludes it is likely a fable; a writer in 1905 decided "that there is no secret." In 2026 Glamis Castle is the home of Simon Bowes-Lyon, 19th Earl, and is open to the public.