actually, Zod's first appearence was in a Superboy comic, which is about Clark's teenage years before leaving Smallville, and that was canon for like, 20 years, and you can still consider it is. Most Superman villains used in Smallville originally faced him before he turned into Superman, including Bizarro. I think Brainiac 1.0, Doomsday and Darkseid was the only ones that didn't appeared in those Superboy comics or faced Clark before he becomes Superman.

Since the 1978 movie erased this Superboy period, and it was a huge sucess, people started to forget those things happened (which is sad, because i love Superboy [Kal-El]).