The legend of the Monster of Glamis relates to somewhere around the turn of the 18th/19th centuries, when a grotesque and bloated monster was born to be Heir of Glamis. Completely misshapen, he had no neck, very small arms and legs, and looked like "a flabby egg", half-human, half-monster. In spite of such deformities he is said to have been immensely strong and is reputed to have lived for nearly 150 years, some people thinking that he finally died in 1921. He lived in a special room at the castle, where he was kept from everybody's eye. His existence was known to only four men at one time, the Earl of Strathmore, his heir, the family lawyer and the factor of the estate. At the age of 21 each succeeding heir was told the secret and shown the rightful Earl. Succeeding family lawyers and factors were also told of the secret, but at any one time no more than four knew of the existence of the Monster. As no Countess of Strathmore was ever told the story, one Lady Strathmore, having heard rumours approach the then factor, Mr Ralston, who flatly refused to reveal the secret saying "it is fortunate you do not know the truth for if you did you would never be happy", a reference presumably to the unhappy state of several Earls of Strathmore during the suspected lifetime of the Monster. Even now it is suspected that the remains of the Monster are still retained in the secret room. Mr Ralston, who was described as a shrewd, hard-headed Scot, would never sleep in the castle under any circumstances. One night, when he had worked late, a sudden snowstorm came on. Pressed to stay for the night he refused to do so and insisted that a path be dug in the snow to his house nearly a mile away. Offering strength to the belief of a hideous monster being born into the family, is a portrait hung in the drawing-room. It depicts a previous Earl of Strathmore with his two sons and an indescribably ugly deformed dwarf.

... a strange creature with thin arms and legs and a beast-like head, described by one percipient as "a hellish combination of all things foul and animal.". This young lady, sleeping in a "haunted tower room" in the castle in the 19th century, was awakened by the strange apparition or creature, which vanished when someone else rattled the bolt on the door before entering the room. Was an underworld or non-human rapist interrupted or frightened away?