How are 化け物, 妖怪, 幽霊, etc. related to each other?

Question

In the past few years, reading light novels and the like, I have come across many different terms for ghosts, spirits, monsters, etc. in Japanese, and I'm wondering how they relate to each other. What are the differences? Is there some kind of Venn diagram that illustrates this? An article that describes the differences maybe? Your help is appreciated.

I will list all of the words that I've heard that fit into this category, in no particular order. I'm sure there are more, but these are the ones I can recall.

• 化け物（ばけもの）/ お化け（おばけ）
• 妖怪（ようかい）
• 怪物（かいぶつ）/ 怪獣（かいじゅう）
• 魑魅魍魎（ちみもうりょう） (I particularly liked this one)
• 怪異（かいい）
• 幽霊（ゆうれい）
• 鬼（おに）
• 変化（へんげ）

First Update: I've grouped synonyms as suggested in the answers below, and added one more.

The answers below provide quite a bit of information, but also leave quite a bit unanswered. In @Kafka Fuura's response, 「怪異」 is defined as 「化け物」 or 「変化」 -- so is it an umbrella term, or are these all actually synonyms?

Additionally, 「魑魅魍魎」 are described as 怪物, but then are further defines as "various types of 化物". Does that mean that 怪物 are a subset of 化け物?

I guess what I was wondering is how these groups inter-relate with each other. Can anybody tell me what the taxonomy of these groups is, or at the very least provide some sort of reference which does so (even in Japanese)*?

_{* One possible such reference is the novel 魍魎の匣, suggested by @Kafka Fuura, but I'd prefer something more detailed.}

Second Update: Since my last update, I have come across more information which is relevant to this question, specifically, The Kaii-Youkai Denshou Database.

According to their introduction, 妖怪 are supernatural creatures (examples given include kappa and tengu), while 怪異 are essentially unexplained, strange phenomena. They also say earlier in the introduction that the terms もののけ, 化け物, 変化, etc. are words which were used to refer to 妖怪 and 怪異. So is it safe to say that these two are umbrella terms which encompass everything else?

Answer 1

The other answers, while containing valid and useful information, are not quite what I was looking for, so I continued to research on my own until I came up with the following taxonomy of these terms, with explanations of their meanings as I have come to understand them and examples.　Note that the meanings of words and the structure of the classifications below is hypothetical as I have tried to divine the "true" meanings of words. In popular usage, I've found that many of these words are simply interchangeable. 化け物, for example, can be used for nearly anything, including extraterrestrials. Also, note that while this list is not extensive due to the broad nature of material covered, I've tried to cover all the major groupings.

• 怪異（かいい）- As @KafkaFuura stated, kaii are irregularities. They are oddities, mysteries, and aberrations. They're strange noises in the night, smells that seem to come from nowhere, and shadows shaped differently than what's casting them. Another example from my personal experience which could possibly be classified as a kaii: have you ever been walking or driving, and just as you go under a street light, it goes out? That's the kind of thing that falls into this category.

• 妖怪（ようかい）- Youkai was originally much closer to kaii in meaning, but at some point came to refer specifically to creatures or things which cause odd or irregular phenomena. They are, essentially, the kaii which have been identified as a specific thing. There are many types of youkai.

  • 化け物（ばけもの）- Bakemono are primarily, as the name suggests, "things which change." They are things which have changed from their rightful form one way or another. Also known as お化け（おばけ）and 大化け（おおばけ）. The following subgroups exist, but caution is advised as their names are often used as synonyms for "bakemono".

    • 化生（けしょう）- The most common type of bakemono is known as keshou, because they "live in a changed state." They can generally not change their appearance, but rather change or are changed from their original state and remain in their new one.

      • 怪物（かいぶつ）- Kaibutsu are ghastly, paranormal creatures whose true form is unknown.

        • 怪獣（かいじゅう）- Kaijuu are the subset of kaibutsu which are animals or beasts.

        • 魑魅魍魎（ちみもうりょう）- This is actually a group of two subclasses: chimi and mouryou, or the spirits/monsters of mountains and rivers, respectively.

      • 付喪神（つくもがみ）- Tsukumogami are items or artifacts, such as swords, sandals, and brooms, which have become alive and aware. This is often said to happen when the item reaches its 100th birthday.

    • 変化（へんげ）- The other class of bakemono is that of the henge. These are creatures which can change their shape via their own form of magic, including foxes, tanuki, snakes, cats, badgers, and many others.

  • 鬼（おに）- Oni are a class all to themselves as broad as any of the others, and include the demons of Buddhism, mountain religions, and many other sources. As in the other answers, they are archetypally huge, hideous creatures with horns and claws, wearing a loincloth and carrying a club. Their skin is often red or blue. According to Wikipedia,

    [Oni] were originally invisible spirits or gods which caused disasters, disease, and other unpleasant things. These nebulous beings could also take on a variety of forms to deceive (and often devour) humans. [...] The invisible oni eventually became anthropomorphized and took on its modern, ogre-like form, partly via syncretism with creatures imported by Buddhism, such as the Indian rakshasa and yaksha, the hungry ghosts called gaki, and the devilish underlings of Enma-Ō who punish sinners in Jigoku (Hell).

  • 幽霊（ゆうれい）－ Yuurei are, in the strictest sense, ghosts, as stated in @Anonymous's answer, though there are many different types. To add confusion to the mix, one major type is often known as お化け（おばけ）.

  • 妖（あやかし）- Often simply called 妖怪（ようかい）, ayakashi include things such as kappa and tengu -- the patently "Japanese" creatures, monsters, and goblins of legend.

This is the answer I was looking for, and only came upon this model after many, many hours of research over the past couple days. However, I also found something else which is worth noting, and which I mentioned briefly above. Popular usage of terms such as 化け物 and 妖怪 are largely interchangeable. Nearly anything can be a 化け物, for example. Whether something is a 化け物 or 妖怪 is often a very confusing issue, and this is why in popular usage, these words can be so confusing: there is no real concept of a hierarchy or structural classification at all.

Answer 2

• 化け物（ばけもの） and お化け（おばけ）are the same thing. They seem to refer to monsters which are native to Japan, such as Kasa-obake.

• 妖怪（ようかい） I believe these are also similar to お化け.

• 怪物（かいぶつ） are usually monsters with fur, or at least that's the impression I get when I hear it. Large, fur-covered, violent creatures that don't seem to think about anything.

• 怪獣（かいじゅう） Same as above.

The next two terms I've never heard before, although that's an interesting compilation of kanji there.

• 幽霊（ゆうれい） are, of course, ghosts. Sadako from the Ringu, she's a 幽霊. So are the mother and son from Ju-on. Of course, when you go and visit your grandmother's grave and think you see her there when you're walking to the grave, that's a ghost too. An apparition of someone (something) that was once alive.

• 鬼（おに） are big, muscly creatures with horns. They often wear tiger striped loin cloths, and carry clubs. Or at least that's how Japanese illustrations show them.

Answer 3

怪異 is "(supernatural) irregularity"

魑魅魍魎 are the demons of mountains and streams. The book 魍魎の匣 goes more into their etymology but the general term means a sort of conglomeration of spirits and demons of nature.

Here are the definitions in my 旺文社　日本語辞典 if that helps.

怪異（クワイ―） 一（名・形動ダ）不思議であるあやしいこと。また、そのさま。「超自然の―」　二（名）化物。変化（へんげ）。

魑魅魍魎（―マウリヤウ） （「魑魅」は山中の怪物、「魍魎」は水中の怪物）さまざまな化物。

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Also for interesting side notes:

The term 鬼 was originally used to describe "those that resisted the state" and were demonized and banished. As in the above answer they are often depicted with loin cloths with colored skin and horns and act as messengers and/or employs of hell depending on the story. I suppose it's possible they they referred to the Ainu or other natives or something.

餓鬼 "gaki" or "brat" is derived from the Buddhist term for "starving ghosts" who fell not into hell but into a semi-realm where they were continuously tortured by hunger, having their tongues ripped out by demons and such.