I've mentioned zombies before, but I wanted to bring up something that has always been a sticking point for me: the use of the word "zombie" in a fantasy game. I've never been a huge fan of it, to the point that not a single one of my NPCs has ever uttered the word. With the way that pop culture of the last ten years or so has been swarming with the things - both figuratively and literally - I find that it's much too easy to ruin the immersive nature of the game by reference to a common term. (Incidentally, I also have this problem with Ktulu* after Steve Jackson Games started the wave of plush toys... even though Cthulu Dice is one of my favorite quick-play party games.)
Before I started playing tabletop RPGs, I ran into this issue. I got around it then by following the lead of the original Night of the Living Dead, and referring to shambling, reanimated corpses as "ghouls". I'm still a fan of this idea, as it's just vague enough that there's no precise definition. A "ghoul" might be a carnivorous corpse, or a roaming undead necrovore. The only issue is that a "ghoul" in D&D terms refers to a very specific type of monster, that is often semi-intelligent and not usually created to serve a necromancer or "evil" cleric.
I've already demonstrated to my players the lethality of even a relatively small group of zombies, although some of them feel that that specific scenario was unfair; but that's a story for another time. Perhaps there's a good way to keep them scary...
* Yes, I spelled it the way Metallica spelled it. No, there isn't a particular reason. I just like footnotes.