Answer: In chapter 10 of "The Hound of the Baskervilles," Barrymore tells Watson about "the other man" who is revealed to be Selden.
Selden was an escaped convict from the nearby prison. Barrymore and his wife had been secretly helping Selden by giving him food and shelter. This had been done because Selden was a friend of Mrs. Barrymore's younger brother, who had visited them in the past.
Barrymore tells Watson that Selden had been hiding out in a small room at the back of Merripit House. However, on the night of Sir Henry's arrival, Selden had escaped from the house and had not been seen since. Barrymore is worried that Selden might have been responsible for the recent mysterious happenings on the moor, such as the theft of food from the Barrymores' pantry. Watson becomes convinced that Selden was the figure he saw on the moor near Baskerville Hall.