Translated from Mizuki Shigeru’s Yokai Daihyakka
What is the difference between Yokai and Obake?
Generally speaking, yokai and obake are two words that mean the same thing. If a distinction must be made, it could be said that obake are those creatures which somehow change from one form to another (the Japanese word bakeru which forms the root of obake means to change for the worse, or adopt a disguise).
Examples are transforming kitsune (foxes) or of the dead people whose lingering grudges cause them to appear as yurei, or a myriad of other shape-changers.
What kinds of Yokai are there?
If you wanted to organize yokai by large categories, it could be said that there are Yurei (spirits of the dead) Kaiju (monsters), Henge (shape-changers) and Choshizen (supernatural phenomena).
A chart would look like this:
Yokai
| Yurei | Kaiju | Henge | Choshizen |
| Examples are hitodama, borei and shiryo, and onryo. The spirits of humans still lingering on Earth. | Animals or insects with mysterious and magical powers. | Anything that can change from one form to another. | Mysterious or puzzling phenomena. |
How many kinds of Yokai are there in Japan?
As a rule, it is said that there are about a thousand different species of yokai. But if you limit it to those that have appeared in pictures or those about which we have some information, then it is really roughly four hundred different species.
But there are many yokai of other countries. Nobody knows the number of worldwide yokai.
What is the largest Yokai of them all?
That is the Onyudo. The body of the Onyudo is as large as Mt. Fuji. However, it is said that even if angered the Onyudo would never cause harm to humans.
Further Reading:
For more Types of Yokai, check out:
6 Types of Japanese Yokai From Showa
When Food Attacks – 6 Types of Food Yokai From Japan
10 Famous Japanese Ghost Stories
百物語怪談会 Hyakumonogatari Kaidankai 
Oct 18, 2019 @ 23:08:39
I don’t know if this blog is even active anymore, but I have a question regarding Humanoid Yokai like Aobōzu as one example, would that yokai still be considered a Kaiju or would Human-like Yokai fall something else?