Thomas Horn Jr., (November 21, 1860 – November 20, 1903) was an American scout, cowboy, soldier, range detective, and Pinkerton agent in the 19th-century and early 20th-century American Old West. Believed to have committed 17 killings as a hired gunman throughout the West, Horn was convicted in 1902 of the murder of 14-year-old Willie Nickell near Iron Mountain, Wyoming. Willie was the son of sheep rancher Kels Nickell, who had been involved in a range feud with neighbor and cattle rancher Jim Miller. On the day before his 43rd birthday, Horn was executed by hanging in Cheyenne, Wyoming.