Mutants
The Mutants are the horribly disfigured results of the Plague -- that is, the nanotechnology-related human subject research that Capek conducted on Mars.
Mutants were once miners. These miners were horribly oppressed by the Ultor Corporation -- Capek's employers. Ultor would often kidnap miners and allow Capek to experiment on them without limit; the resulting beings were unrecognizable and extremely violent.
It is theorized, but not confirmed that these creatures are somehow related to Red Faction: Guerrilla's Marauders.