Some from Command and Conquer:
From the Tiberium series:
-Tiberium: This alien substance can kill you in little time without treatment, for which there is none;
-Visceroids: Lumps of mutated humans fused into a single entity, hard to kill (at least in the old games), and will spit Tiberium at you;
-Any of the mutants you find in the later levels of Renegade: they are fast and kil you in few hits.
From the Red Alert series:
-Chrono Leggionaire: He basically erases you from time, as if you never existed;
-Terror Drones: Small machines developed for dismantling vehicles that were converted to destroy enemy vehicles and kill enemy soldiers;
-Desolators: In RA2 they carry a radiation cannon that kill infantry in seconds. The RA3 version of the desolators, they are terminaly ill people put in suits (almost like a Big Daddy from Bioshock) and carry chemical launchers.