Fighting Chocobos goes back much further than just 13, or 12, they appear in battle as early as 7 if I remember correctly, though I forget if they fight back or not. Also, Chocobos have been popping up in combat as allies for some time as well, to see them on the other side of the battle should not be so foreign.
More recent games have had Chocobos as common enemies, this is talking about adding them in our most recent expansion, in a setting where it makes sense, a rare opportunity in this games lore surrounding Chocobos. I think it would make sense to add them, not only in game, but also as they have been doing it more often in recent games, and the fact they are a breed of enemy which would be new, unique, and probably easier to do than brand new enemies. Another thing that makes this appealing is that if added, it would open doors for some more interesting things, Chocobo Knights with Chocobos in combat, BST being able to charm them, BLUs being able to learn from them.
