From the Super Mario Wiki
Wheeere's Yoshi?! (also known as the Yoshi Shell game) is a minigame found exclusively in Super Mario RPG: Legend of the Seven Stars, which the player may select to play, doubling Mario's party's experience points or coins.
To win this game, the player watches the Yoshi Egg in which Yoshi hides, then selects it after it spins around with two other eggs. There is a black creature resembling a Fuzzy in one egg, and a pink bird in the other. The Fuzzy makes the party lose all wagered items (experience points or coins). The pink bird represents a tie, and the player keeps the original amount.