Is Acnologia a human or a Dragon? In Fairy Tail, it is shown that he was a Dragon Slayer who had transformed into a Dragon. But, who was he before becoming a Dragon?
As Quikstryke's answer said, Acnologia was one of the first Dragon slayers. The more he fought with dragon magic, the more he became dragon-like.
In a recent manga chapter, it is revealed that
Igneel and the other dragons were "alive/dead" inside the dragon slayers, and as the dragon slayers matured, the dragons inside them prevented the dragon magic from consuming them and turning them into dragons like Acnologia's case.
He was definitely a human. Not sure if you're reading the manga or watching just the anime, but its pretty evident he was a human who was taught dragon slayer magic. They said as he killed more dragons he started transforming into one. Dragneel and the other dragons prevented this from happening to the current generation dragon slayers.