User blog comment:PUNCHURPREGNANT/Gray and Silver Predictions/@comment-59.93.129.56-20140113095710/@comment-1359322-20140114070108

God knows....actually Mashima knows how Acnologia transform into a Dragon. It could be that he used a magic from the Book of Zeref, absorb other elements from Dragon and Dragon slayers, bath in their blood overuse of DF all this combined help in his evolution.