Mercenaries (or Bio-Engineers) are one of the enemies present in Unreal and Return to Na Pali. Their weapons are forearm-mounted machine guns and missile launchers. They are frequently portrayed as enemies of the Skaarj Empire.
With most measuring 6' tall and weighing 250 lbs., Mercenaries are not to be trifled with. They are described as aggressive, agile opponents who primarily rove the galaxy in search of wealth. They use bio-engineered suits.
There may be a humanoid beneath all of that circuitry, but the only way to find out is to blast your way through it.