There are exactly three versions of each monster class; weak, medium, and strong. In the case of Chocobos, this equates to Yellow, Black, and Red. There are a few exceptions to this (i.e. Demons have only two types, and Apanda have only one), but there is no class that gets more than 3. I suppose you could hack the game and palette-swap one of the existing chocobos to green, but that's it.