“What're YOU lookin' at? If you wanna challenge me, you have to bring Cookies!”
One day, Boshi defeats all the other Yoshis in a race and becomes the champion of the Mushroom Derby. Boshi then calls himself boss of the Yoshis. Under his reign, the races of the island are cancelled. In fact, Boshi himself is the only one allowed to use the racecourse. Although the other Yoshis disliked Boshi's rule, they felt obligated to obey the champion of the derby.
During the events of Super Mario RPG: Legend of the Seven Stars, Mario travels to Yo'ster Isle via the Pipe Vault. Once there, Mario meets up with his old friend Yoshi. The green Yoshi delivers the bad news of Boshi's reign, and Mario decides to help free the island. Yoshi recognizes that Boshi has a weakness for Yoshi Cookies. Yoshi asks a friend to loan him some cookies and Yoshi and Mario confront the boss of the island. If Yoshi can win a race against Boshi, then he will become the new champion of the Mushroom Derby (as such, Boshi will lose his authority). Boshi, who would never decline a wager with Yoshi Cookies on the line, agrees to a race.
During the race, Mario rides on Yoshi's back, helping him defeat Boshi. To beat Boshi to the finish line, Yoshi has to move to the beat of the derby's music. As the music plays, the player must alternate between and with the music's tempo. If the player presses the buttons too fast, Yoshi does not move. As the song plays, the beat may stop or change. The player must continue pushing the buttons to the original beat, not the new beat, however. Additionally, if Mario has any additional Yoshi cookies, he can feed them to Yoshi, giving him a boost of speed.
Yoshi wins the race and becomes the champion of the Mushroom Derby. The other Yoshis congratulate him and demand that he become the new boss of the island. Yoshi declines the offer, stating that the races should be free for everyone to participate in, not controlled by a boss. Yoshi's words even reform Boshi, who agrees to participate in fair competition.
Boshi, ever the loner, still likes to stick to himself, but he has learned to enjoy the spirit of competition. Proving he is still a good runner, Boshi manages to win a race every once and a while; however, he is not the dominant racer he made himself out to be.
The ending cinematic of Super Mario RPG: Legend of the Seven Stars seemed to show that Boshi had befriended Croco after the latter had apparently moved to Yo'ster Isle.