User blog comment:FairyRaven/Fairy Tail Chapter 324: The One Who Closes The Door, Review and Commentary/@comment-112.205.41.120-20130323032321/@comment-84.113.52.218-20130324005327

Well... it is only predicted that there will be demons and dragons present in the "dragon king festival". The prophecy (or whatever it is) doesn't say wether the demons fight on the dragons, the humans or their own side. So I guess theoretically it's possible that Zeref has them battle the dragons.

Then there would also be Mirajane and Elfman. In their take over they could be mistaken for demons.