User blog comment:IamJakuhoRaikoben/Chapter 412: Dance Unto Ishgar, Review./@comment-99.120.149.4-20141208151639/@comment-7418318-20141208153415

That Acnologia used to be a Human and Dragon Slayer, but at some point his body was transformed into that of a Dragon is established fact.

Though it isn't known why or how he transformed exactly. The two beings who taked about it in the story had theories, but weren't completely sure either juding from their used words (or at least that's how the translations sounded).

One theory goes that Acno transformed by himself after bathing in the blood of many Dragons he slew, while the other theory blames it on Zeref.