The original inhabitants of Exo III, who moved underground as their sun faded and were destroyed by the androids they created.
The Old Ones were the native people of the planet Exo III. When their sun dimmed and the surface became frozen, they moved into vast underground caverns and built android servants to help them survive. In time the androids came to see their makers as a threat and destroyed them, leaving only the machines and their technology behind.
In the episode What Are Little Girls Made Of?, Dr. Roger Korby, the missing fiance of Christine Chapel, was found living in the caverns with Ruk, the last surviving android of the Old Ones. Korby had used their technology to build new androids and to move his own mind into an artificial body, and he planned to replace key people across the galaxy with android copies, beginning with James T. Kirk.