When I first learned about Acnologia having been a human that turned into a Dragon due to Zeref and then more recently about the Etherious demons I began to wonder "could Acnologia himself be one?"
While Acnologia is a dragon I don't think its a stretch when you consider Zeref was involved that Zeref made him a Dragon but also turned him into an Etherious Demon. Its likely that the power Acnologia gained as a result was able to dwarf Zeref's own which is why even Zeref himself fears Acnologia.
There's some evidence to this I believe I've found, thanks to the words of Atlas Flame. He said that Igneel was unable to destroy E.N.D., while E.N.D. is now confirmed to be a Fire Demon the fact he's stated to be the strongest demon (its likely the deceased dragons like Atlas Flame never learned Zeref had a role in creating Acnologia) and even Igneel struggled to win shows dragons probably struggle against most Etherious demons normally.
So imagine mixing an Etherious Demon with a powerful Dragon, the result would be even worse. And since it seems going by Tartarus' actions that E.N.D. is loyal to Zeref and not as powerful as him, so if Acnologia had ever found Tartarus himself they would have likely all been wiped out even with E.N.D. summoned.
Acnologia could just be a dragon who due to his actions as a human with all the dragons he slayed and dragon blood he bathed in became extremely powerful when Zeref used black magic to turn him into one, but comparing how much Fairy Tail's members have had to struggle to defeat atleast one of the Etherious demons in Tartarus and how even all 7 of the Dragon Slayers were unable to kill the 7 weak dragons Future Rogue had brought through the Eclipse Gate I don't think its impossible for Acnologia to be a Dragon/Etherious Demon hybrid.
~ What do you think about this? Does it seem possible, think there's more evidence that I missed or something that makes this less likely, or maybe you have a different theory entirely? Feel free to give me your ideas on the subject.