i heard somewhere, maybe from an LS member or forums, that the dragon on the airship isn't neccessarily dead but sleeping. suspended animation have you. and the other dragons are trying to gather enough aether to re-animate it. i doubt its nidhogg on that ship unless the event from the opening movie happened a really long time ago.

i do believe that it is bahamut on the airship and the garleans tried to capture it and "tame" it but it backfired.