Boshi is a Yoshi who only appeared in the game Super Mario RPG: Legend of the Seven Stars for the SNES. Being a Yoshi with an attitude, He is distinguished from the other Yoshis by his sunglasses, spiked collar around his neck, and the fact that he doesn't wear the same shoes as the other Yoshis. Before Mario came to Yo'ster Island, Boshi was the best racer in the races titled the Mushroom Derby on the island. One day when he beat all of the other Yoshis on the island in a race, he declared himself boss of all the Yoshis, and allowed no one except him to race in the derby unless they brought him Yoshi Cookies, Boshi's favorite food.
Once Mario came to the island, the main green Yoshi told Mario about Boshi. Mario agreed that he would help Yoshi beat Boshi. After asking one of Yoshi's friends for a few Yoshi Cookies to spare, Mario and Yoshi challenge Boshi to a race with the Yoshi Cookies. Boshi, not being one to turn down Yoshi Cookies, accepted Mario and Yoshi's challenge and the race went underway. After the race with Boshi, Mario and Yoshi won. Yoshi is now the new champion. When the other Yoshis decided to declare him the boss of the Mushroom Derby, he declined and said that its free to everyone including Boshi, who later learned his lesson.
After you beat Boshi in a race, you can talk to him anytime to race again with him and the other Yoshis on the Island if you have Yoshi Cookies to wager.
In Japan, Boshi's name is Washi, which is similar to both Wario's and Waluigi's names in theme. Most fans consider Boshi to be Yoshi's rival in the same manner as how Wario is Mario's and Waluigi is Luigi's.