Culex sidequest
Using the "Shiny Stone" on the sealed door in Monstro Town reveals a hidden boss named Culex. Contrary to popular belief, the Culex character is not from any particular Final Fantasy game and instead simply represents a sort of generic "Final Fantasy-ish" boss.
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Culex with Earth,Wind,Fire,and Water Crystals
The Earth, Wind, Fire, and Water Crystals that accompany him originally appeared in both Final Fantasy II [IV in Japan] and Final Fantasy V [which was never released in the US]. During the fight with Culex, the Final Fantasy II boss music plays in the background. When Culex is killed, the original Final Fantasy 'victory fanfare' plays, and the Final Fantasy 'introduction' music plays during Culex's conversation in the overworld.
Square Soft, the company that co-produced Mario RPG, is also the producer of the Final Fantasy series, Thus the appearance of Culex