Speaking of the most basic, and brainless, members of the undead, in the Thaumatology universe, you can just tell that some zoologist got their claws into zombie research early on in the proceedings. Zombies are divided into four classifications. Yes, four. You’re wondering how anyone can get four classifications of zombies, right? Well, they cheat.
Class 1 zombies are the sort you expect. Take one, relatively intact, corpse, reanimate it with magic. Class 1s are not bright, but they are loyal, fairly strong and expendable. Annoy a necromancer enough and he just might turn you into one of these after he pulls your still beating heart out of your chest.
Class 2s are dead bodies which have lost their soul, but somehow retained their spirit, or regained it. Far more independent than Class 1s, the 2s can result from failed attempts at lichedom, occasionally failed vampire turnings, and also situations where someone comes back to exact revenge; the classic Revenant.
Class 3s are not even undead. There are various spells, alchemical potions, and even herbal concoctions, which can entirely obliterate a living person’s volition. They act pretty much like zombies, but they need to be fed and they are no tougher or stronger than a normal human. Doing this to someone is illegal in most civilised countries, but it is actually used as a punishment for some crimes in one or two places.
Class 4s are the comedy version of zombies, skeletons. No one seriously creates these because it takes too much power and Class 1s are far more useful, but a few necromancers with more power than sense have skeleton guards because they think it looks cool and shows off their awesome might.
To date, no one has found a disease which reanimates corpses, so there’s no need to stock up on food and paint your windows white. (D’Oh! Wrong holocaust!) Ghouls now… ghouls might be a different matter. Those things are freaky…