I have given a lot of thought to the manner of beings found in the Castle Acheron campaign. What varieties of being are there? How are they classified? How did they come about?
There are certainly animals, plants and other forms of life, similar in nature to those found in our world. There are also beasts that are not found in our world, but are otherwise similar to natural animals. "Above" those are spirit beings, which include spirits clad in flesh and disembodied spirits. It is in this category that humans and others that can be considered people, (like hobbits and orcs, if I decide to include them) are found. These beings are primarily spirits, but inhabit animal bodies. There are also ascended beasts that are similar, but different in that they are created only late in an animals life and then only under specific conditions. These would include the kitsune, tanooki and nekomata spirits of Japanese myth. The spirits of people survive the deaths of they're bodies. Most leave this world for other planes of existence such as the Lands of the Dead (Hades, Hel, etc.), or the realms of they're Gods, but some remain in this world. There are also various spirits of nature, such as those found in animistic religions. Then there are spirits that do not have animal bodies at all, but may have "psuedobodies" of different natures. These include beings we might call faeries, but this category may also extend into the third category I have yet to discuss. Elves, dwarves and gnomes would fit under this category.
This third category also includes beings we might call angels, demons or gods. They are not spirits in the same manner as those of the second category, but of a different sort. I'm not really certain about the basis of the distinction, but I do think that beings such as demons and gods should be distinct from men. However, in this campaign I will make no distinction between demons, angels and gods. Though, some "angels" may in fact be beings created by "gods" to serve them. The distinctions between "gods", "angels" and "demons" will largely be a matter of the mythologies held by mortal beings. However, it's not that they are completely wrong, as they're mythologies will have some basis in fact, reflecting much of the actual relationships between the deities. However going further into this subject would be beyond the scope of this post. Perhaps I might call this category "celestials".
There may also be a fourth category of abberant beings that don't fit any where else. Similar to the aberration creature type of 3rd Edition or the aberrant origin of 4th Edition, and like those inspired by the Chtulhu Mythos. I haven't read any actual Lovecraft stories, but only things inspired by them like D&D and Warcraft. I find the creatures and beings of the Mythos to be fascinating, but do not agree with Lovecraft's world view. So the influences of the Mythos on my campaign are likely to be superficial. It should also be noted that my "celestial" beings include certain elements that make them similar to beings from the Chtulhu Mythos like alien forms and to some degree mentality. Some are at least superficially human-like but others are less so.