Powerful telepathic beings who punished Kirk's landing party for trespassing by forcing them to relive the gunfight at the O.K. Corral.
The Melkotians are a powerful telepathic species. They warned ships away from their space with a buoy that spoke directly to each crew member's mind.
In Spectre of the Gun, Kirk ignored the warning, and the Melkotians sentenced his landing party to death by recreating Tombstone, Arizona, from his own memories, casting them as the Clanton gang facing the Earps. Spock realized the bullets could kill only if they were believed real and mind-melded with the others to remove that belief. When Kirk refused to kill his attackers, the Melkotians welcomed peaceful contact with the Federation.