Fat zombie guy.

Slow, stupid and slaps like a umm zombie and likes brains.

Thin zombie guy [where are the girl zombies?]

Not as slow as his fat friend, just about as stupid and has a bit more of a sting with his slaps.

Zombie commando (dodgy arm).

Zombie Soldiers from hell, this creature is equipped with a long tentacle like whip that can reach quite a far length of distance, and won't give up without a brutal fight.

Zombie commando (chaingun).

Similar to their zombie commando twins, but with powerful chain guns that can spin off rounds at you and do you in very quickly instead of using a long tentacle. These foes are no monsters to take lightly.

Z-sec zombie.

UAC security guards turned into mindless zombie security guards with pistols, and is no friendly zombie.

Z-sec zombie.

UAC security guards turned into mindless zombie security guards with shotgun and better equipped armour etc, more dangerous than their pistol counterparts, they are more dangerous to confront.

Z-sec zombie.

UAC security guards turned into mindless zombie security guards with machineguns and yet even better armoury, these guys know how to strafe and can be quite annoying so you might want to deal with them off quickly.

Flaming zombie "Bernie".

A flaming zombie that happens to trudge around quickly, "Bernie" is a very relentless foe that will trail his flames everywhere and follow you around very quickly, so make sure that he's dead ASAP.

Chainsaw zombies.

A bit slow, stupid, and his mission is to tear you apart with his chainsaw naturally. Don't let him get close to you or you'll be transformed into nothing but giblets within a matter of seconds, kill him only from medium or far ranges.

An Imp, nasty jumpy thing that can really slap [those zombies should take some lessons] and throw fireballs at you.

Doesn't take much to take down but it moves fast and can really dish out the damage before you get a shot off.

Believed to be related to the Demon [of the pinky flavour]

A mechanical rear ends makes a lovely clanking noises against the steal floors, giving you an idea of where they are.  If they get close to you, you'll be knocked around all over the place before you know it and will be lucky to get a shot off in the right direction.  Take them out before they get close!

The Trite,

A spider like monster.  Seen lowering itself from the ceiling and believed to attack in groups.

Tick. 

Similar to Trites, they deal damage by biting and exploding, but in packs can become very pesky. A few pistol rounds from rather long or medium distances will take care of these little buggers.

Cherub. 

Basically a baby demon with wings and claws, like an insect sort of, buzzing around here and there. Easy to pick off from long to medium distances as long as they stay out of your close area of range.

Maggot.

Two headed, amazingly fast but fairly weak crawling beats, these buggers can be taken down with a single, nicely ranged and done shotgun pellet.

Wraith.

Relentless, dangerous, close to imp creatures with extremely large claws that will cut you apart, and these monsters can teleport closer to you if you are further out of their range, creating a rather tougher battle with these buggers. Some rounds from a machinegun or better from long distances will serve you well against these enemies.

Lost soul.

Annoying flying head type thing with sharp teeth.  Known to attack in groups.

Cacodemon.

Floating, but slow, beasts that can deliver major quantities of hammering damage. Their mouths open up to offer belching fire balls, you can take them down pretty easily with a shotgun or better, but these demons can be dangerous and annoying, so be careful.

Revenant.

Semi-transparent, Skeletal creatures with home targeting rocket propelled launchers on both of their shoulders, also can swing you apart in melee combat. This creature is also very fast and deadly, so pick it apart with a chain gun or better.

Arch-Vile.

A slow yet very dangerous enemy, the Arch-viles will shoot a instant stream of long fire at you that will do lots of damage, so these creatures from hell should be taken down quickly.

Mancubus.

Slow, fat creatures, blasting balls of fire out of it's flame launcher built hands/arms, and at a melee range can take you down very quickly, these are very powerful monsters not to be taken lightly.

Hell Knight.

Drooling, sharp teeth, tall and huge beast quality featuring demons, slow creatures but the one of the most dangerous next to bosses. Can slash you apart with their long claws or throw balls of plasma at you which deal quite a lot of damage. Use caution, these monsters are not to be underestimated.

Craig, a newly discovered monster [end of game boss?], with several special attacks; such as the 'hyperactive let's annoy everyone' move and the 'death by a thousand pokes' move.

Should this monster be encountered the best course of action is to use the thermal nuclear strike gun, or run like hell, since there's no such thing as a thermal nuclear strike gun, running is a good idea.