What's a good monster to get my PCs running away?

[Fiffergrund]

My current campaign has all kinds of variants on the zombie theme, but none of them are 1) actual zombies or 2) behave precisely like zombies.

Some have spider climb abilities. Some spew corruption from their mouths. Some emit fog. Some have no internal organs, but continue to function and look like humans (except for the homicidal mania).

This really puts people on edge.

(Also, I need to do this thread necro thing more often. It's always nice to revisit topics after an additional 8 years of experience!)