Get all the Flowers in all the stages including the special stages to get access to a final level that is located in the main hub.
Silver Yoshi
Defeat bosses 5 to 8 in the Boss Tent.
Yoshi Hut
The Yoshi Hut is a place located in the main hub on Craft Island where you can change your Yoshi's appearance based on the Yoshies that you have saved. To unlock the Yoshi Hut, simply collect all 5 Yarn Rolls in any level.
Bronze Yoshi...