Slave Mooks: There are some species and polities that like to keep these around (often AIs or other disfavored “not-people” in the eyes of galactic bigots, although that tends to come with special risks; see Robot War), and some people – well, for ultimate values of complete bastards, people – do this on a small scale by shoving puppet implants into prisoners or even civilians, but perhaps the worst example is the one I hint at in Alien Invasion – spray some self-replicating indoctrination neurovirus into the atmosphere of the planet you’re attacking, then recruit all the enthusiastic mooks you’ll ever need from the newly rewritten local population.

Fortunately, none of the people with the technical ability to do this on a mass scale have yet had the urge to rocket themselves to the top of the Galactic Atrocities List and probably get half the Worlds coming to get xenocidal on their asses, but the thought does keep strategic defense planners up at night worrying.