User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-32252462-20180520135615/@comment-30924344-20180520155311

Bahamut is not a dragon in original mythology. It is a gigantic fish (or whale) that lies deep below, underpinning the support structure that holds up the earth. DxD's Bahamut is a fish also so DxD stays true to its depiction.