Imps and demons, ghosts and spirits, humes and humanoids, Farmer John getting you off his land by poking you in the butt with his pitchfork, elementals and golems, unknown things and creatures of your own custom design, etc.
That some off the top'o my head.
Good luck with this, sounds like it's gunna be interesting.
Killerzzz
These are all good lists of monsters, and I do know what most of them are, but what makes them so special to you? How could they be in the game as more than just another guy-with-an-axe?
Imps: Little devil-like creatures, often red but can be other colours (green, sickly yellow). Known to have a tail with a triangular point at the end, and sometimes has a little pitchfork. Their size vary from a little less than a foot tall, to up to three feet in hight (maybe four but...).
Demons: They are sometimes seen as big versions of imps, but are often anything that looks like it is spawned from hell. That is to say, a demented distorted creature that may or may not follow any laws of how an earthly being should look, and commonly has many uneven (or out of place) razor-sharp teeth.
Ghosts: Humanoid entities that are the walking souls of the dead, on earth. Transparent, often unaware of the fact that they are dead, they may or may not be the type that likes to screw around with people to scare them. They may or may not be able to affect solid objects around them.
Spirits: More often than not, they aren't humanoid. Transparent, they are perhaps in the form of whisps or fogs, and may try to posess objects or even people. It is spirits of demons who are prone to humanoid posession. They may also temporarily take on creepy demon-human-like shapes. They may or may not be able to affect solid objects around them.
Humes: A short form for 'humans'. Be they common men, wizards, amazons, knights, sorceresses, bandits, archers, etc., thats the humes.
Humanoides: These are the creatures that have the basic shape of humes: technically elves, orcs, imps, some demons, halflings, some faries, etc. are all humanoids. You may even have bird-men, satyrs, goat-men, wolf-men, giants, any other animal-hume mix, some gargoyls, etc. that fall into this category. As far as I can tell, humanoides need to be organic (not walking statues, for example).
Farmer John: It would drive a man insane to even try to think of explaining this creature, so I shall not. <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" /> All I can say is that, if you step on his land, you will never return... without having him give you a nice few jabs with his hay-fork!
Elemental: A creature that is spawned completely from the very energy of the elemnents. They are made from lightning, fire, water, ice, earth, plant, air, etc. They are part of nature, and are offspring of the elements caused by a non-understood reaction, or by some unexplained rage. The elementals can have a small or large control over the elements themselves, using them at will.
Golem: A summoned creature, made at the sacrifice of a material. They are sometimes viewed as lesser formes of elementals, but they are actually nothing like them other than that some are made of the elements. They are more then less often created by dark summoners or necromances or sorcerers. They can be made from the earth beneath the summoner's feet, from the metal of his weapon or armor, from his own blood (a very binding ritual, that has them share their pain and gain), or a mix from these things and a piece of the elements. Golems are almost always created by a summoning, and are slaves to the ones that created them.
Unknown things: I don't know, they're unknown. <img src="/ubbthreads/images/graemlins/silly.gif" alt="" /> Perhaps that things that moves at the corner of your eye, or the boogeyman. Or, you can call it 'uknown entity' or something of the sort and have some indistinct shadow with eyes where ever you want.
Creature of your custom design: Well, thats up to you. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Killerzzz
P.S.: Killerzzz: always taking it a step too far. <img src="/ubbthreads/images/graemlins/rolleyes.gif" alt="" />
P.S.S: Yup, this is all from me, this info, collected and redirected from years of videogames, horror movies, fantasies, etc. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />