There are a number of monster classes, the most common one by far being demons, although zombies have also been seen in released media. The monsters belong to one of these classes:

Necros - The creators of undead and masters of necromatic powers.

Examples: Flesh Eater, Grotesque

Beasts - The least cunning demons with the most animal-like behavior.

Examples: Carnagor, Stalker

Spectrals - Mysterious ghost-like beings only partially existing in our world.

Example: Orbile, Soul Reaper

Primus - The intelligent and fearsome dukes of hell, often more organised then most other demons. Able to build and use custom equipment.