The way I understand it is that they're all separate and independent beings, but still the same. For example, Akatosh, Auri-El, and Alduin are all different aspects of the same oversoul, Aka, which is the being that jump started Time when it came into existence. So the Nords see Aka as Alduin, and Mer see Aka as Auri-El, but both of these beings exist independently of each other. The Nords don't see Auri-El as a dragon. To them, Auri-El is Auri-El, just like Alduin is Alduin and Akatosh is Akatosh.
It's less a case of "Jupiter is Roman name of the Greek God, Zeus," and more a case of "Jupiter and Zeus are the same being perceived by different cultures, but are their own entities and can interact with one another."
Edit: If the answer to my earlier question was yes, then Aka also has a whole lot written about them, but I'm not sure if the various aspects on their own have enough information to rank as high in the list. Alduin and Akatosh might.