Unused sprites exist for all seven Koopalings within the game, which suggests they were at one time intended to appear as in-game bosses. Interestingly, the order they were intended to appear in is identical to the order they appeared in Super Mario World (1990).
In the US version of the game, Petey Piranha is mistakenly referred to as "Boss P. Plant" in the Glossary (a direct translation of his Japanese name, "Boss Pakkun"). This was fixed in the European version.
One of the enemies in the game, Starfish, is a reference to Starfy of the Legendary Starfy series. The enemy bio states that he is rumored to be a prince of a kingdom, which Starfy is, and all of the Starfish's sprites are Starfy's with sunglasses added on.
This is the last Mario game where Jen Taylor records any new voice clips for Princess Peach and Toad. Mario Hoops 3-on-3 (2006) and Mario & Sonikku at Pekin Orinpikku (2007) would reuse voice clips from other Mario games where Jen Taylor voiced Peach.