Yes but still it makes you think that there are other GF besides the ones that you possess and Dark Bahamut might simply be one of those GF.

The theory that it could even be Selphies old GF makes sense if you think about it...Selphie had a GF but for some reason that she can´t remember due to memory lapse she abandoned him or lost him, this made Bahamut turn into a "dark side" feeling hatred torwards the humans that he felt that betrayed him.

Another theory that i have similiar and maybe more crazy is that, Dark Bahamut was created in the Deap Sea Research Center as a clone of the original Bahamut, but the experiment went wrong and the monsters and GFs that were being used as an experiment turned loose and destroyed the lab which explains why the lab is in ruins when you encounter it.
Then Dark Bahamut remained hidden like most GFs until he was found by Ultimecia who used is hatred torwards mankind to turn him into her Guardian.

And finally my third theory involves cards...when you check the boss cards it appears bellow a combination of monsters, i wondered if this could mean that the bosses were created after there was a junction between monsters. Which would mean Dark Bahamut was originated between a juction of an Hexadragon and a Grendel.

It might seem strange to some people but who knows maybe one of this theories is true.