Here's a pic of Midardsomnr and his children. The middle one is midgardsomnar....can't make out the rest.



We know that Middy was the 1st dragon in the world. He had 7 children. Meracydian dragons worshipped Bahamut as being "one of the 1st brood" that spawn their clan.

It was said that we'll meet his 7 children throughout the story, so it's possible that Bahamut was a child one of Middy's kids.