The walking dead.

Or undead as they are also called. A sentient being, who's body is animated though quite dead. There are actually rather few things out there that fall under this category.
Vampires ofcause, but that's so big a subject that it warrents it's own post or maybe several.

Zombies, while not sentient, are the generic undead. There's a real craze these days about zombies, almost as big as vampires really. But is also another can of worms I won't open here.
First off, there are no such thing as zombies, atleast not as they are potrayed in movies and games, virus transmitted via bite, that kills people and raise them as man eating, feral corpses. and thank god for that. Because if they did, the end of the world senario we see in movie after movie would be pretty much a given.
Something that spreads that fast and aggressively, would wipe out humanity in no time.
That is not to say, there there are not things that would qualify as zombies. But they are vampire related and I'll elaborate on it in my post about the blood suckers. But sufficient to say, is that vampires can raise a corps as an obidient slave.
Some of these raised corpses sometimes retain enough sentience to be feral like dog and will, if not controlled, attack and eat anything living it comes across. Though it's victims won't come back (luckily) But I guess that's a scant consolation, with a corps gnawing on you. But they're most likely the course of the zombie myth.

Another form of undead are Liches, mages who's become so strong that when their bodies expire, the magic keeps their soul anchored to their bodies and gives it unlife.
They are immensly rare, I've only heard of two in my life time and while not dangerous per say, they're suppose to be a bit eccentric.
But I must admit that amoung mages, they are like the sort of rumor that everyones heard about, but noone really believes possible or will admit to have taken seriously. Sorta like penis extention. Everyone knows a guy, who knows a guy who heard about someone who meet one.

Now, the final form of undead are actually a little misunderstood. Graveyard squatters, corpse eaters, discount vampires. Ghouls have very few flattering nicknames.
Ghouls are species of undead, who have a really bad reputation, personally I think it's undeserved, but I'm biase on the subject, but I'll get to that.
The theory is that Ghouls are an abiration of vampires (just don't mention it to a vampire or anyone assosiated with vampires, unless you want to insult them grossly) I have to admit that they share several traits, but not many.
There are several reasons for the Ghouls bad reputation, first off is their looks. When someone becomes a Ghoul they go through a transformation and it is not a pretty thing. The skin pulls tight over the body and the body dries up, leaving them a distorted husk. Their eyes sink in and their teeth grow to fangs and their toughs grow long and rough, for better digging out marrow. Their nails lengthen into claws.
That's the obvious reason why Ghouls are hunted so feriously, because they look like genuine monsters (and some are, but that's another matter)
The one, that really marks them are their diet, like a vampire has to drink blood, a ghoul has to eat corpses, human corpses and for some reason they take their dinner seasoned, thus their tendency to hang around graveyards were they can dig up a properly aged dinner.
This is the reason I don't get the hate levelled at them, as a rule they don't kill humans they eat, their chosen victims died of natural causes. I agree digging up your relatives and eating their bodies arn't exsactly a good thing, it's better than them taking live humans and making a meal of them.
All the same they are treated as pestes, hunted ferousiously as monsters and this is odd as most Ghouls are quite harmless, unless you're dead and as they retain their intelligence from their human life, the ones that have also retained their sanity are as such not bad people. Most just wanna be left alone.

Ofcause there are exceptions to the rule, the ones that ruin it for everyone else. Some people, who turn into ghouls lose their minds, either go insane or were insane to begin with or go feral. These bad seeds will hunt and kill and pickle their victimes (ghouls can't eat fresh meat) or even try and force people to become ghouls (that was one ugly job)
So if you end up facing a feral ghoul, be careful. These are dangerous bastards, cause Ghouls are strong, fast. agile and tough and don't have the classic weaknesses of vampires. You can stake just about every organ in their body and they'll just keep truckin. A ghoul will regrow what ever he eats off a corps. so the only sure way to kill a Ghoul is taking off it's head.
They don't like sun light, but mostly couse it dries them up and hurts their eyes. Alot of hunters underestimate Ghouls the first time they encounter them, most are so lucky they meet a ghoul that chooses flight over fight. The ones that meet one that doesn't usually don't get the chance to meet one a second time.

A ghoul is created when a Ghoul feeds him some of it's own flesh, (not as romantic as the vampires, I know) The ghouls flesh is poisonous and the human soon dies, the body then reanimates and come back to unlife. over a periode of time, the person change into a ghoul and must saten it's hunger for corpse flesh or go feral, as it's body will shut down it's mind to get what it needs. this is where most of the insane ghouls come from, because sane people won't want to be turned into a ghoul and tend to go insane as their new reality dawns on them. and if you did choose to become a ghoul, then you most likely had a few loose screws to begin with.
Alternatly, If a ghoul takes a recently dead person (doesn't have to have killed them themselves) and then shoves it's flesh down it's throat, the body will come rise and over time change into a ghoul. This is the most common way for a Ghoul to be turned. At first they will be feral, not much brighter than dogs, but over time their sentient mind will reassert itsel. Most, if not all of their former life forgotten. These have a much better chase of accepting their new existance.

My personal experience is that Ghouls have been given a bad hand in unlife and mostly don't want to make trouble and just be left alone. So they look scary as hell, eat corpses and have a breath that could knock out a rhino. But I still think they're alot nicer than your average vamp and don't deserve the rep that they got.
But again, I owe a Ghoul my life, so I'm not as unbias in my oppinion as others.
But that's a story for another time.