Ogres are a subhuman race and enemies in Ground Missions. They are mostly encountered in valleys. It is suggested that Ogres did not exist as a naturally occuring species and instead were products of Three's experimentation.
At some point between years 998 and 999, Three developed the first Ogres as apart of her experiments and let them go into the wild. This action angered One who ordered that Three cease her experiments and destroy them entirely, however these eradication efforts ultimately failed.
During Zero's campaign against her sisters it is suggested that Ogres were no longer seen as pests and were now being used as troops in the Intoner Empire's forces. The species survived the deaths of all the Intoner sisters and presumably went on to become mercenaries in the Empire.