Pern

Pern was colonised by settlers from Earth who wanted to return to an agrarian society with a low level of technology. Shortly after the first colony was established, the settlers were devastated to discover that their chosen planet was not a safe place. Pern is subject to periodic attack from space by the destructive thread, a spaceborn spore that destroys everything organic it touches. Unable to retreat from the peril, the settlers developed methods of combatting the thread. "Holds" were dug into the cliffs for them to live in, and Pernese scientists "upgraded" a local reptilian lifeform, the fire-lizard, into full-sized flame-breathing telepathic dragons, who were able to sear the thread before it could fall to the ground. The dragonriders became a vitally important and highly respected profession, living in Weyrs, and as time progressed, the settlers forgot their Terran origins.