Since we haven't seen the reincarnation of the demon king assuming he has reincarnated or has a long life. It might actually be him.
As in you might be right that it was the demon king who wounded the black dragon
Based on the convo between Zak and Fia, he's not king yet. His age is 0 in terms of dragon age, or 12-13 years old in human age, meaning it's just an infant. This leads us to suspect he is probably the child of the previous Black Dragon King.
Also, there was mention of how the beasts are acting weird because 'one' of the Beast King went missing. It hinted that there are three beast king, with the black dragon just one amongst them; each has their own territory which they reign over.
Okay so the black dragon is tye king of beasts and the strongest but in the first chapter before she saved him he was wounded so badly that he become a trembling weak and pitiful little bird, so who did that to him? Did he lied when he said he's the strongest monster? Or did he face the demon king?