how did folkmetal start?

Skyclad was probably one of the first acts that really blended folk music with metal. Before that there were mostly just influences, they didn't go all the way with it...

But I guess one could argue that acts like Lez Zeppelin and Deep Purple incorporated some folk-music-sounding parts to metal/hard rock in the late sixties already.
 
I don't really think Quorthon is using folk-influences in his music, just conceptous. He really started a whole new genre; the so called viking metal genre. To be called viking metal these days, you really have to add Bathory-influences :)