There is a optional hidden boss in Monstro Town named Culex, who is an homage to the Final Fantasy series. His appearance purposely breaks the style in Super Mario RPG to reinforce this homage.

To fight Culex you must go to Moleville and purchase fireworks, then trade them at the little mole's Pur-Tend Store for the Shiny Stone. Use this stone on the sealed door in Monstro Town. You'll end up in a battle with Culex, and if you beat Culex you'll get the Quartz Charm.

Culex's battle, victory, and post-battle conversation themes are (respectively) the boss theme from Final Fantasy IV, the victory theme from the Final Fantasy series, and the Final Fantasy main theme. Also, the Elemental Crystals that Culex uses are the same as in Final Fantasy and Final Fantasy IV.