Isn't Bahamut a primal?
That puts him into a slightly different category than something like Midgarsormr or Nidhogg. The latter being physical, though powerful beings, rather than the former's summoned state. That's why the Allagans captured all the Meracydian dragons to keep Bahamut summoned.

That says to me he's either not Midagard's son or he was, was killed or died, and was subsequently summoned?