I'll admit my two thoughts on it.

I don't think it is Bahamut, it'd be an unusual choice for S.E to let one of their trademark monsters die before the game even began, unless of course the story eventually moves into a point where he's resurrected once all the primals are defeated.

Though personally I'm hoping it's a different dragon and at some point there's a cut-scene where it fights against Bahamut, a giant dragon brawl would be an awesome sight to see through this graphics engine