At the core, Youkais are

basically phenomenons

I'll give you an example

Good morning!

The "Yamabiko" is a youkai

The Yamabiko are the phenomenon of hearing your own voice echoed at you in the moutains

This phenomenon then took shape as a youkai named the Yamabiko

Yahooooh

With time people began to think the youkai was the cause of the phenomenon

The youkai then wasn't thought as the phenomenons itself but as what causes it

And this is how the youkai known as Yamabiko came to exist