From the Super Mario Wiki
When the player decides to stop here, they will meet Bowser and his cohort, Koopa Kid. Their quest, Bowserstein!, involves the player choosing one of Bowser's monsters and dueling it. Choosing the very scary monster is not advised, for all that comes is Huffin' Puffin and it's children. This makes Bowser embarrassed, and makes the player leave. Choosing the scary monster will pit them against Naval Piranha, and play the Duel minigame Koopa Kurl. Defeating Naval earns the player the right to keep the mini-game.