User blog comment:Red Eyed Raven/just a quick thought/@comment-3424593-20131212052154/@comment-24066070-20131212072547

It seems Zeref turned Acnologia into a dragon. He also created many demons, whether he created them out of thin air or turned animals/humans/objects into demons is unclear. The dragons do seem to have something to do with Zeref. I think Zeref was at some point an all-powerful madman who went around doing whatever he liked to whatever he encountered. He made some demons and some dragons, killed heaps of people, invented some spells and wrote a book. The book of Zeref may just be his diary, containing stuff like "I made a demon through this method. I made a portal through time and imma call it eclipse gate"

However, all of these connections are mostly speculation for now.