The first thing which might tip off a reader about the mysterious beast which is pursuing and killing members of the Baskerville family is the title of the story-- The Hound of the Baskervilles. Even if one did not know the title of the story, several characters are convinced of and mention the (ghostly) murderous dog.

Early on in the story, we read about the death of Sir Charles Baskerville, who was found surrounded by giant paw-prints in the grass. Dr. Mortimer, who shares the death of Sir Baskerville with us, suspects that his friend may have been killed by a curse on the Baskerville family. Centuries prior, Sir Hugo Baskerville was supposedly attacked and killed by a giant dog. Since then, there has been a folktale of a hell hound which haunts the moors. All these factors considered, in addition to recent reports of a great, glowing dog being spotted on the moors, naturally direct the characters as well as the reader to suspect there may really be a killer hound.

Unfortunately, Holmes and Watson do not uncover any hard evidence of the dog (such as fur) until they encounter it head-on. Nonetheless, Holmes has been able to work out the motive and means of the real culprit.