In Paper Mario: The Thousand-Year Door, the secret boss beyond the Pit of 100 Trials is Bonetail.
The Pit of 100 Trials is an optional dungeon beneath Rogueport’s sewers. Mario must fight through increasingly difficult battles across 100 floors, with occasional rest areas and rewards along the way. Completing the challenge is not required to finish the main story, but it is one of the game’s toughest postgame-style accomplishments.
Bonetail appears on the final floor. The enormous skeletal dragon is the older sibling of Hooktail, the dragon boss fought earlier in the story at Hooktail Castle. Bonetail uses powerful attacks, can restore its health, and may inflict several status effects, making the encounter far more demanding than most regular battles.
Bonetail is commonly confused with Gloomtail because both are dragon bosses, while the Shadow Queen is the game’s main story antagonist. Atomic Boo is another optional boss, but it is not the reward encounter at the end of the Pit. The original GameCube game was released in 2004.