Seth Shelton

 was an outlaw in the Indian Territory who was wanted for being a killer.

In 1880, he tried holding Bill Potts hostage. Bill was saved by United States Deputy Marshal Bass Reeves and the scout Joey Two Trees, who had Shelton arrested and intended to take him back to Arkansas to stand trial. While Reeves, Joey and the Doctor took off, Seth's brother Frank Shelton arrived at the Reeves' caravan and freed Seth. Seth and Frank waited for the Twelfth Doctor, Reeves and their friends to return to the caravan so they could kill them. (COMIC: The Parliament of Fear)