Fenrir

[Anonymoose]

he existed far before these six elemental primals did

"These" is an important word in that sentence. In the Third Astral Era, different elemental primals existed. Belias being one, for example, whose egi is a spitting image of Ifrit's. The egi look similar presumably because they are both drawn from the same element (or, through my lens, the same essence of the land, Inferno). Belias was revered as a Demon King and Ifrit has horns and uses Hellfire, so maybe they also looked similar as deiforms, too; I'm not sure. Anyroad, Allag summoning was born when Meracydia summoned primals to resist Allag conquest, which brings me to...

Bahamut himself isn't an elemental primal

Meracydia was successful in fending off the Allag with the early primals of that era (presumably elementals, possibly inclusive of Belias); Xande died without bringing some of the great powers under the yoke. Louisoix states that Bahamut was summoned "in the twilight of the Third Astral Era," meaning that the unstoppable Allag advance that made the dragons summon Bahamut was after Xande was resurrected and became so determined to see his dream of a world-empire come to fruition the contracted with the Cloud of Darkness to harness the power of the Void.

The energy present in the corporeal realm is nothing compared to the energy present in the Void (kind of like how much energy is bound in material atoms in the real world). Harnessing the power of Darkness depleted even the energy stores sustained by the Crystal Tower, so Dalamud was sent into the heavens to harness the sun's energy to make up for it... until Xande unleashed all of its energy at once in an attempt to let the Cloud of Darkness into the corporeal world to undo everything back to Void, ushering in the Fourth Umbral Era and setting off a cataclysmic earthquake that destroyed the Empire.

Because Bahamut's appearance was late into the Era's collapse (after earlier primal summoning and introduction of Void corruption), it's really no different than the appearance of Moggle and Phoenix coming after Ifrit, Garuda, etc. I'm not saying that means my theory is true, of course, just that I don't see a conflict (*yet*). I also wouldn't be surprised to learn that Alexander, Ravana, and Bismarck aren't strictly of the the six elements, either. (After all, Bismarck is historically aquatic).