Super Mario RPG and its remake have a secret, true final boss that some players will never even encounter. This is a classic RPG tradition – many games in the genre, including several other Mario RPGs, have hidden bosses separate from the ones that appear during the course of the main story. Some games have multiple secret bosses that can be unlocked throughout the campaign, while others can only be fought after watching the credits roll. Super Mario RPG only has one, but he more than makes up for it by being incredibly difficult.

https://screenrant.com/super-mario-rpg-hidden-secret-boss-culex-explained/