In 2254, James T. Kirk visited the planet on an undercover cultural survey mission. He discovered that the planet was inhabited by a group of primitive humanoids who live in tribes known as the Hill People, which are led by female shamans known as the Kahn-ut-tu. Because of the primitive nature of the civilization, the planet was protected by the Federation's Prime Directive. (TOS episode: "A Private Little War")
One of the most dangerous creatures native to the planet is the Mugato, a predatory ape-like creature that have a lethally poisonous bite. A local form of planet life is the Mako root plant, which is the only known treatment for a Mugato bite. (TOS reference: Star Fleet Medical Reference Manual)
In 2267, the USS Enterprise arrived in orbit of the planet to conduct a follow-up survey. Transporting down to the planet, a landing party discovered that the Hill People had split into warring factions, with some possessing flintlock firearms, which were beyond their technological level. Further investigation revealed that the Klingon Empire had made contact with the planet the previous year and was arming some of the natives with the firearms.
Kirk decided that the planet must not fall under the control of the Klingons, and decided to violate the Prime Directive by contacting the other factions and offering to help them in their fight, by providing them with flintlocks to "even the scales". (TOS episode: "A Private Little War")
This resulted in an arms race, with the Klingons and the Federation providing their factions with increasingly advanced weapons. A civil war soon broke out in which a large portion of the population was killed. (TOS video game: Starfleet Academy)
Eventually, the native humanoids wiped each other out. In 2376, former Starfleet officer Ensign Janos, having lost his sentience, was brought to Neural by Captain Mackenzie Calhoun to live out his life there. He was last seen joining a pack of wild Mugato. (NF novel: Stone and Anvil)