Just need to chirp in here but.
It isn't overtly mentioned, however, a couple of NPCs in the game (most specifically one of them is outside Darkhold) mention that the Garleans are ALSO moving their army against the dragons that assault Ishgard, and the guy outside Darkhold ponders if that isn't because they fear that the dragons have a primal of their own.
I'm a firm believer that that's a Checkov Gun (means that "if it is mentioned, it will be used") and more than likely Bahamut will end up being the Dragon Primal.