The problem is, as cool as it would be, the sheer volume of enemies in the game would make this too difficult and time consuming to implement so late after launch, especially if it's more for lore reasons (based on your mention of the FFXII bestiary). Having said that, there already is a basic bestiary provided by SE - albeit not in the game itself, but hosted instead on the Lodestone (found here), which I think will have to suffice.