Adarnians

Adarnians were a fairly non-threatening and harmless sapient xenos species that was placed under Imperial Protectorate, meaning that they were not to be exterminated by any rowdy crusader fleets. They were mostly ignored by the Imperium until it was discovered they could be used to produce Rejuvenat drugs to reverse the aging of the upper crust of the Imperium. This resulted in the brutal genocide of the entire species just to drain them for this purpose. They are now extinct, with the last of their kind being used to keep Archmagos Ezekiel Sedayne alive a little bit longer to meet Belisarius Cawl.

This is similar to the Stookies from Judge Dredd.