Deities we see in Fantasy settings, especially those like D&D, are quite a bit different than what exists in the historical records of our world. Sometimes they're almost clear cut, like Quetzalcoatl, while other times they're dramatically different, like Bahamut. I'll explain a bit, and maybe we can discuss things a bit further.

Quetzalcoatl is a Mesoamerican deity, they actually show up in the religion of at least 3 Mesoamerican cultures, only one of which is the Aztecs of which most people think the great Feathered Serpent originates, but Quetzalcoatl has been traced as far back as the Toltec civilization, last I checked, and their domains have been similar yet different among each culture.

Bahamut is a bit of different beast all together, quite literally. While what most think of as his counterpart, Tiamat, is quite clearly dragon-like, Bahamut is something most Fantasy (and D&D) buffs aren't in full knowledge of. Bahamut and Tiamat are Mesopotamian deities, that much is true, but the thing about Bahamut is that he was never actually a dragon, while Tiamat most clearly was. Bahamut, according to what has been gathered about Mesopotamian culture, was actually a Giant Fish. In fact, one of his titles I managed to uncover is "The Fish That Supports the World" and the reason for that name is actually kind of interesting. According to his lore, Bahamut is a fish of immeasurable size that carries the world upon his back. So yeah, in some cultures, like Japan, the world is carried on the back of a giant turtle, while in Mesopotamia the world was carried on the back of a god who just happened to be a fish.