Adarnians

Adarnians were a fairly non-threatening and harmless sapient xenos species that was mostly ignored by the Imperium until it was discovered they could be used to produce Rejuvenat drugs to reverse the aging of the elites of the Imperium. This resulted in the brutal genocide of the entire species just to drain them for this purpose. They are now extinct.

This is similar to the Stookies from Judge Dredd.