_cf said: Many youkai "awake" fully formed [Citation needed].
How the hell do we cite oral tradition?? :D
Anyway, that's certainly true for tsukumogami (objects that attain sentience on the 100th anniversary, like Kogasa), I'd probably be hard-pressed to find an artistic representation (maybe try this interpreting this one as Kogasa coming to life from her umbrella: http://danbooru.me/post/show/678712/) or an account about the birth of a youkai. So let's just assume that youkai simply "come to be" in their final forms, complete with personality, shall we?
_cf said: Many youkai "awake" fully formed [Citation needed].
This page does a nice job of showing how this worked out for Meiling.
Explain Chen.
There were things that weren't beasts, too.That's why I'm sure I was born there that day.Sometimes when I was hungry I would catch and eat them.Rather, I was already there and already just as I was the moment I was aware of anything at all.They would be called "youkai," I think.I don't remember arriving, or anything I might have done before.There were moving things around me that were different from me. Afterwards I learned they were called beasts.