The youngest of the Earp brothers in the Melkotian illusion of Tombstone in Spectre of the Gun, who kills Chekov in a quarrel over a saloon girl well before the gunfight.
Morgan Earp, brother of Wyatt and Virgil, appears in the Original Series episode Spectre of the Gun, played by Rex Holman, who later played J'onn in Star Trek V: The Final Frontier.
In the Melkotians' recreation of Tombstone he is described as the brother who kills on sight. Chekov, cast as Billy Claiborne, is courted by a local girl named Sylvia whom Morgan regards as his own. When Chekov steps in to protect her, Morgan shoots him dead in the street.
The death puzzles Spock, since the historical Claiborne survived the day, and it leads him to conclude that events in the illusion can be changed and that only belief makes its dangers real. Chekov is found alive once the test ends, and Morgan fades with the rest of the town.