In my first blog post, I predicted that Acnologia was an Ethernano Dragon Slayer. In my third, my assumption, based on the fact that the markings on his body in both his human and dragon form are similar to the ones on Mard Geer in his Etherious form, was that he was one of Zeref's demons. Both of which are unlikely. Here's my new theory. I hope you like it.
See any similarities? Besides their markings? Like their heads maybe?
My new theory is that Acnologia is an Etherious-human hybrid, a bastard born of Mard Geer. 400 years ago, Mard Geer Tartaros, one of Zeref's demons, attended a human brothel and... well, you get the picture.
One of the harlots came to him 9 months latter with a baby, claiming it was his. Mard Geer, feeling insulted of such a thing, killed the woman and was about to kill the baby until Zeref stopped him. Zeref pointed out to Mard that the baby's markings are similar to those in his Etherious form. Killing it would've made him a kin slayer. Mard begged for forgiveness and asked what he must do. Zeref told him to care for the baby and raise him as his own. Zeref named the baby "Acnologia Tartaros" or for short, "H.E.A.T." (Human Etherious Acnologia Tartaros).
When the child matured, he was merely a human. He had no an Etherious form. The markings were his only prof he had of being half Etherious. When the Dragon Civil War began, Acnologia joined wanting to learn the magic of Dragons.
By this time, E.N.D. was created. When E.N.D. was forming Tartaros (the guild), he saw Acnologia as a potential member, being a Dragon Slayer. He offered him to join if he could defeat every member including himself.
Acnologia did so. He beat every member including his own father, Mard, who was surprised he didn't even hesitate. When his opponents entered Etherious Form, he entered Dragon Force. But when it came down to E.N.D., Acnologia was beaten within an inch of his life. E.N.D. spat on him saying that he's not even half Etherious and while he may have the power to slay a dragon, he doesn't have the power to slay an Etherious. E.N.D. then revealed he was using Acnologia to test weather Curse, his new invention, was superior to Magic. He was never giving him a chance to join the guild.
Acnologia asked for his father's help but he simply replied, "You're no son of mine". Acnologia, angered and humiliated, took his anger out by killing dragons, enemy and alley both including the one who taught him, going so far as bathing in their blood.
He eventually grew scales and fangs. Thinking something bad, he begged "Lord" Zeref for help. Zeref did so. He put a spell on him that separated his dragon form from his human form, giving him the ability to switch back and forth between the two like it was an Etherious form. His ability to use Dragon Force was no more because of this.
Being half Etherious would explain why Dragon Slayer Magic doesn't effect him and why his dragon form looks somewhat comparable to an Etherious. Getting his ass kicked by E.N.D. would explain why he's scared of him so much.