The Thraal were a species wiped out by the Olabrians.
Circa 500 BC, a Thraal ship accidentally landed on Olabria in a garden of Olabrian joy-luck crystals. This sparked off a century-long war, during which the Thraals and their home planet were destroyed by the Olabrians. All that was left of them were records of Olabria. (PROSE: Ship of Fools)