The humanoid inhabitants of Beta III, who lived for six thousand years as placid members of the Body under the control of the computer Landru until Kirk talked the machine into destroying itself.
The Betans appear in the Original Series episode The Return of the Archons. Their society looks like a quiet town of an earlier Earth century, but every citizen is absorbed into the Body, a collective calm overseen by robed Lawgivers. The peace is broken only by Festival, which begins at the Red Hour and lets the population run riot.
Landru was once a living leader who ended an age of war and built a computer to carry on his rule. The machine kept his knowledge without his wisdom. The starship Archon was lost there a century before, its crew absorbed or killed.
In 2267 Kirk and Spock, helped by the underground members Reger and Marplon, reach the computer and convince it that it is harming the Body. It burns out, and a sociologist is left to help. Lower Decks returns to Beta III in No Small Parts, where the Betans have slipped back into obeying Landru.