I personally kept all my Aeons and Tidus as default, but a friend of mine... he just went crazy and renamed everything that could be renamed.
This might be a little hard to understans for someone that doesn't understand Swedish.

Tidus = QeFg (just random letters i guess)

Valefor = Dämien (Damien)
Ifrit = Fläjm (Flame)
Ixion = Fläsch (Flash)
Shiva = Schälä (Tjäle... tror det stavas så)
Bahamut = Bälseböb (Belsebub)
Yojimbo = Häselhöf (Hasslehof..... don't ask)
Anima = Räelig (Rälig = slang for very bad looks)
Cindy (the fat one) = Fläsk Fia
Sandy (the tall one) = Däägan
Mindy (the small one) = Litet Fä

As told above, if you're not from Sweden you will probably not know what they mean. And, mind you, we DO NOT write like that. The names are greatly exaggerated, but if you know swedish then you should know what it means.