Karkas:
A huge mass of green Zs. They are controlled by a central hivemind (of a different colour, so you can see which one it is… or not?) and will all deanimate as soon as it is killed. They can range in size from 5 to about 20 Zs.
They operate as a cohesive swarm, all stuck together, never breaking off from the group unless an obstacle forces them to. They will attempt to engulf players, making taking them on in the open very, very dangerous.
To balance their sheer strength of numbers, and their ability to block off an entire street to the player, they would be slightly slower and weaker than normal greens, and thus possible to escape.
This would incentivise carrying some sort of explosive or fire weapon, for a quick strike at the centre of the group.
Quantum:
Wraith-like creature that can cross through walls, and is unlikely to be harmed by bullets (like skeletons). Is not slowed by terrain. Weak attack but very high evasion.
Whisper:
Wraith-like creature that can produce refractions of itself. These are indistinguishable from the real thing (unless perhaps to a high perception check) but do no damage, and have only 1 HP. The creature can produce them at an alarming rate, however, and it can be difficult to find the “real” Whisper amongst the 10 or so refractions it can maintain at any one time.
Would definitely encourage melee, as the speed of reproduction would mean that the player would waste a lot of ammo taking care of the clones, and should focus on finding the “real” monster before using his/her gun.
Mutter:
Wraith-like creature that can teleport over short distances. Behaves much like the “fast” enemies (fast zombies, wolves) in employing hit-and-run tactics, except it does so by teleporting from 6-9 tiles away and back again.
Tendril:
An angry tree. Why do we not already have angry trees?