Captive Tiresians.
Tirolians, sometimes called Tiresians (after the largest city on Tirol) were a humanoid race native to Tirol, a moon of the planet Fantoma. Tirolians are nearly identical to humans from Earth, save for a handful of cosmetic differences in hair, skin, and eye colors. The triumvirous Robotech Masters evolved from the Tirolians while in search of immortality and a perfect race. The clone Masters were notoriously harmful to the natural-born citizens of the planet, even leaving them as bait for the Invid as they ran for Earth. (Robotech II: The Sentinels, Robotech II: The Sentinels)
The Robotech Master's experiments in Tirolian physiology and cloning also led to the further development of the Zentraedi race.
Behind the Scenes
In the original Robotech cartoon, the Robotech Masters were never referred as Tiroleans. It wasn't until the cancelled-Sentinels TV series that they were given a name and a homeworld.