I think the Fayth Anima wound up inside would have once been the home of a full out water based aeon. As for lack of Ramuh... Exodus has it more or less right. The devs on FFVIII had a vision of the GFs in that game of being much more animalistic and Ramuh just didn't fit so they went with Quetzalcoatl. I haven't read anything really that states why in FFX they used Ixion though.
