Thousands of years ago, the world was populated by a green skinned race of insectoids called the Nauians. Their advanced civilization detected a meteor that was going to hit their homeworld and catapult it into an ice age. In order to survive, the Nauians created a highly complex underground shelter in what would be later known as Mount Idyll where they were placed in suspended animation until the next eclipse where the gravitational field would awaken them.
Not all Nauians managed to reach the shelters and some remained on the surface of their world when it was engulfed by an ice age. During this time, a great cataclysm occurred that destroyed the world's moon, scattering its remains as a planetary ring around Pollux V which prevented the machinery from awakening the Nauians.
By the 23rd century, the world was colonized by settlers from the Federation, with its membership consisting of adherents of the Acolytes of the Stars. Their colony uncovered many artifacts of the planet's past but they were unaware of the Nauian race. As they expanded and developed a mine at Mount Idyll, the ancient Nauian machinery's defensive systems activated which created demonic images to the settlers in an attempt to frighten them away from the sleeping chambers.
In the 2260s, after increasing reports of demons were made to Starfleet, the starship USS Enterprise was dispatched to Pollux V to learn what was the mystery of these creatures. Upon arriving at the planet, they discovered the doorway into the sleeping complex and managed to reactivate the sonambutron machinery which awakened one of the Nauian's who explained his species plight. As a reward for their assistance, the Nauians agreed to share their worlds with the settlers and also wanted entry for his people into the Federation as they believed they had much advanced technology to share. Captain James T. Kirk agreed to make an application for membership for the race and departed the world, allowing the Nauians to awaken on their world once more.
In addition to this, those Nauians that remained on the surface degenerated into a cat sized insects called Silotis and were the largest native life form on that world. (TOS video game: 25th Anniversary mission: "Demon World")