Commenting on this old thread because I actually have some insight for you. There is a decent amount of original rock/punk/metal Korean music, but most of it is performed in English. But that style of music just isn't as popular in Korea. I spent several months in Korea last year and it wasn't easy to find local rock music at first.
Two bands off the top of my head that do perform in Korean are Method (metal) and Harry Big Button (hard rock).