The undead. The shamble around the place, are nailed to things in eternal torment and like to eat brains. They'll chuck lumps of their own poisoned flesh at you so be sure to blast them to bits to stop yourself from getting overwhelmed. Luckily, you've quite a few weapons that are good at dismemberment - experiment.
Undead meat - in a can! They don't throw gibs at you, but will try to hit you with their rusty sword. Their corroded armour leaves them resistant to many forms of damage that'll shred a regular zombie though, so be sure to keep a few grenades spare.
A demon of the netherworld versed in magic. Unlike previous incarnations they have more than one trick up their sleeve - caution is advised. Despite this they're still made of flesh and bone - your weapons should, on the whole, remain effective.
Bleagh. Combined out of lumps of zombie flesh into a janitor / medic amalgamation, this blob will barf venomous bile at any interloper when angry, or else roam the halls devouring any scraps it can find and reconstituting them into zombies. Some weapons also have an unwanted effect - don't be surprised to see him swallow a grenade, only to spit it back at you.
The big guy should need no introduction. He has learned some new tricks over the eons however, all the better to smash unwary slipgaters with. Firstly, he has a much better control over the arcs of electricity he can produce, able to hit at greater distances and earth the crackling flux off of nearby objects. Secondly, their are the slightly more advanced versions of the Dimensional Shambler who never devolved and lost their ability to dimension door. At will. Yes, they can teleport where they wish in response to damage, to surprise you or because you're trying to escape.
A floating netherworld monstrosity that likes to lay its eggs in your face. It'll launch itself at you and try to latch on - this should be avoided. It's allergic to liquids, so having a bath if one grabs you is probably a good idea, in more ways than one. It tends to drift about out of view and try and take its victims by surprise.
They're back, and armed to the chipped brown teeth. Chainsaws, grenade launchers, rocket launchers, flak cannons, nail guns and frying pans. The lovable cannibal brutes should provide hours of hours of fragging fun.
Bound by and controlling various magics, you'll want to keep your distance from these deadly foes. Even that won't keep you out of reach of their fiery summonings, so be sure to send plenty of ammunition their way.
Everyone remembers this organic buzzsaw from their first steps in the world of Quake, usually ending with them lying on the floor whilst it hacked away at their still warm corpse. They're a bit more agile this time around and have a couple of new moves. Something tells me they'll be learning even more new tricks in the future though.
There's something nasty in there.