At some point in the game, they say that the fayth all used to be Summoners - I don't remember where, but I'm pretty sure that's where it was from. I'm just guessing that they were a 'different kind' of summoner, unless Zanarkand had its own stockpile of Fayth, which seems strange since imprisoning the soul of a living person seems kind of extreme. My guess, unsubstantiated at this point, was that the Zanarkand Summoners could manifest their own spirit as the Aeon they're associated with. For example, the fayth of Yojimbo declares, "I am the blade of vengeance... Yojimbo." It's like he actually *is* the aeon Yojimbo, rather than just the source for that Aeon.

The Final Aeon's a different matter, though... Hopefully someone who's read the translated Ultimania will be able to clear this up