I tried to find all the information about his past, but it wasn't even hinted at in the manga. Unfortunately, the author didn't fully reveal Acnologia, so I had to keep that name. But I checked the wiki and find alternative. Puroton Acunorogia — in RŌMAJI, not Japanese. So that i can safe canon.

This version of the character is in the year x300, before the dragon slayings and the war between the two races. Here, he's a doctor in his hometown.