User blog comment:Real Godisme/Fairy Tail 100 Years Quest Chapter 2/@comment-30338537-20180727085336/@comment-7418318-20180727115651

Come to think of it... the only Dragon Slayers that Acnologia counted among the Dragons and hunted were those that either had an actual Dragon inside of them for some time and those that had Dragon Lacrima inside of them.

He didn't seem to be aware of Irene until the moment he saw her.

Maybe that's the answer to this riddle.

Though I'd still have prefered for this story to have some other mystical beast instead of Dragons. Hyrda, Sphinx, Gods, Giant living trees or whatever.