So I'm writing a research paper, and I was interested in knowing which are the most famous? There are hundreds, if not thousands of different youkai so of course I can't write about all of them. xD

I remember:
Kitsune
Tanuki
Yuki-onna
Kappa
(that umbrella guy with one leg)
Kotodama


Thank you very much for your help, and I'll be coming back every once in a while for more info.

Also, what happened to that wonderful library with pictures? I miss that.

Well, most famous in the western sense, I guess you mean? ...there are some youkai that would be considered "famous" among Japanese people that foreigners have never heard of...
In Japan, it would probably depend on region as well. Kappa are particularly famous in areas in the Touhoku region and (I think) in Kyuushuu.
My (somewhat bias) guess would be Kitsune are the more "famous" - or at least, there seems to be many more stories about them, and such a variety of different kinds of foxes (both "good" and "bad", the different colours and tails, etc.) in Japanese folklore in comparison to other youkai (at least, from what I've read, that is). If ever you need help with info, you know where to find me ^^
and being back in Australia is so not fun... I wanna go back to Osaka already ;_;

Also, tengu, bakeneko and oni seem to be popular, but they're not on your list QotS! XD

Ahh I knew about those, just forgot to write them down at that moment. xD

And yeah, I'm actually more interested in the most well known in the Japanese sense.

Oh, and is there a difference between Bakeneko and Nekomata?

Bakeneko is a cat youkai (normally evil, can transform (hence the name), and sometimes, or mostly from my stories, eat people), nekomata was a family cat that reached such an age and wisdom that its tail split in two and it acts somewhat as a guardian for the family.

Oh! That makes sense. I heard that one of them was capable of necromancy too, but I guess that must be the Bakeneko.

The two seem to get jumbled up a bit.

Ah, do you have any books with info on them that you can recommend? Or is it mostly oral tales?

Well, there's a superstition that if a cat jumps on a dead body, it comes back to life, so cats are kept away from people that have died.

As for books, I'll have a look for you, or at least what mentions it. Mostly, youkai are a little like "general knowledge", so they turn up in a lot of different works (like a dragon might in a Western story). Like, there's the "Nansou Satomi Hakken-den" (which I love), that is really, really long and one of the characters' father was eaten by a bakeneko, that then transformed into him and controlled the village or something...
If you want to look yourself, you might be able to find bakeneko info if you search for info from Senshuu (what is now south Oosaka (Izumi region)) - there were a couple of stories from there. Also, I think Okayama Pref. (north area - formerly Mimasaka) as well.
But there's heaps of really old stories - it's just a matter of it they're in Eng., I guess ^^;. I could translate stuff for you if I have the time ^^

The big thing with youkai in Japan (especially in literature) is that they don't always fit these neat classifications, and there's a lot more youkai that never seem to be mentioned outside of Japan.
Hihi, Chou-keshin, Yamabiko, Nurarihyon, Mouryou, Mujina are a few that I haven't seen in English texts (that I've read), but everyone I asked about them knew what they were.

By far, though, Kitsune and Tanuki (and Mujina) are probably the two that have a lot of feeling attached to them - that people recognise quickly, etc.

Oh yeah! I remember Mujina, I really like those guys actually.
I'll make sure to do some more research this weekend, thanks a lot for your help!

You will most likely find it easiest to find academic information on the kitsune. When I was taking Japanese thought and looking for material I could use in my paper regarding Shinto, I was being driven bats by finding so much on Inari and Kitsune, but so comparatively little about water (and my focus had been on water, as that's what I had been wanting to write about).

Thanks! I definitely should include some tsukumo-gami in my paper.
Does anyone remember the names of the sandal and paper lantern versions?