Optional Superboss

An optional superboss is a boss in a Final Fantasy game that is both ultra-powerful (usually stronger than the final boss) but also optional, in the sense that the game can be completed without defeating it. Optional superbosses are usually highly incidental to the plot, if even that, and exist primarily to provide a challenge to hardcore gamers.

Final Fantasy VII

 * Ruby Weapon
 * Emerald Weapon

Final Fantasy IX]

 * Hades