User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-79.115.73.90-20150804104231/@comment-24834479-20150805211336

It was stated that the Leviathan were created first, but this hasn't been discussed much by the writers yet. It is vey possible that we will explain this when we get further into writing the season.

Again, there is no definite origin written for the pagan gods yet. The writers have many theories, and will most likely reveal, or hint to their origin this season.