Yo'ster Isle is an island south of the Mushroom Kingdom where many Yoshies live. The island's first appearance was in Super Mario RPG: Legend of the Seven Stars for the SNES. On the island, the Mushroom Derby takes place where Yoshies race to the finish. For most of the time, a Yoshi named Boshi was the champion and claimed that he ran the island. When Mario visited the island, Yoshi told him that Boshi was trying to rule the island. After this, Mario vowed to help Yoshi win the race. Once Yoshi did win against Boshi, Boshi learned that what he did was wrong.
After defeating Boshi you can mount Yoshi and race Boshi and two other Yoshis, or let Yoshi race without you. You can also bet on the race. If you talk to Boshi, he will tell you Yoshi's chances at winning and allow you to wager Yoshi Cookies in a winner-take-all race. Also, out of a nest of Yoshi eggs to the north, sits a baby Yoshi. If you feed the baby Yoshi enough cookies, he'll become Fat Yoshi and give you items depending on the amount of cookies you give him.
In Japan, Yoshi's Island is known as Yoster Island. The English translation of Super Mario World simply retitled it Yoshi's Island. It is unknown if it is the same island as Yoshi's Island since the Japanese version had the name Yoshi's Island in the game of the same name.