If I recall correctly, it was outright stated in game to be based upon the thoughts of and image of both Bahamut and Midgardsormr. The Middy part is kinda odd though, since outside of the Warrior of Light (and Omega, for obvious reasons) nobody is really supposed to have an inkling of what his Firemane looked like afore it was extinguished.

Though that does make me think, what are the chances of Midgardsormr ever reclaiming his glorious mane of flame? Surely it's a part of his body, and not something that he's truly lost for good in sooth.