"The Old Ones" was the name given to a particular race that lived on planet Exo III by one of their creations, an android named Ruk.
When the surface of Exo III began to cool, its people moved underground, sacrificing their open environment for the possibility of survival within dark caves on the planet. Roger Korby discovered evidence confirming a theory he had: that freedom of movement and choice produced the Human spirit – as the people of Exo III moved underground, they exchanged freedom for a more mechanistic culture.
Eventually, they developed the technology to construct androids. The best of these machines were remarkably capable: physically strong, mentally agile, capable of functioning without guidance for centuries. The Old Ones continued to improve their machines, eventually giving them the ability to feel emotions. Perhaps their greatest technical achievement was the android duplicator, a mechanism that could manufacture a copy of a living being, including his memories.
Giving their androids emotions turned out to be a mistake, as the machines became frustrated with the illogical, inferior beings that had created them. The Old Ones grew afraid of their creations, and began to turn them off. Their survival threatened, the androids overcame their programming and destroyed their builders.
When Korby arrived in 2261, Ruk was still tending the ancient machinery, and had been doing so for longer than even he was capable of remembering – many centuries, at least.
According to Ruk, all of the Old Ones were killed. In 2266, Captain James T. Kirk visited Exo III, attempting to contact Korby. Ensuing events led to the destruction of Ruk and all of the androids created by Korby. The ancient records, Korby's notes, and the android duplicator survived. (TOS: "What Are Little Girls Made Of?")