The Earp gang was a faction of 19th century law enforcement officers that included brothers Virgil, Morgan, and Wyatt Earp, as well as Doc Holliday.
In 1881, a famous gunfight between the Earps and Clantons occurred at the OK Corral in part of both side's attempt to fight for control of the town of Tombstone, Arizona.
This event was reenacted in 2268, after pertinent information was pulled from the mind of James T. Kirk (whose ancestors had settled the Wild West) by the telepathic Melkots. (TOS: "Spectre of the Gun")