The Thains were a peaceful humanoid race. According to one source, they became extinct circa 6000 BC. (PROSE: Placebo Effect)

In the 30th century, a Thain city led by Valda was attacked by Kleptons, who captured many as slaves. The First Doctor, John and Gillian arrived, helped the Thains repel the invaders and freed the ones who had been captured. (COMIC: The Klepton Parasites)

By the 40th century, the Kleptons still hated the Thains, even though they had died out ten thousand years beforehand. (PROSE: Placebo Effect)