Morph bubble

Morph bubbles, also known as metamorphosis bubbles, are floating bubbles found on Yoshi's Island in Super Mario World 2: Yoshi's Island (and its reissue) and Yoshi's Island DS. When a Yoshi touches a morph bubble, the Yoshi turns into a different form, usually a vehicle that resembles the Yoshi itself. Some transformations are temporary, while others last until the transformed Yoshi hits a Yoshi Block. If a Yoshi is transformed into a temporary form, the Yoshi must hit a Yoshi Block to turn back to a normal Yoshi, or pop another morph bubble to stay in that transformation a little while longer. Also, if a Yoshi fails to touch another morph bubble or a Yoshi Block before the transformation time runs out, the Yoshi automatically goes back to the exact location where it first touched the bubble and it reverts to its original Yoshi form.

In Yoshi's New Island, morph bubbles are replaced by Whirly Gates.

Vehicles
The vehicles marked with an asterisk are not in Yoshi's Island DS. In Super Mario World 2: Yoshi's Island, there are sprites for three Yoshi forms that do not appear in the game; these include a Yoshi plane, a Yoshi mushroom, a Yoshi tree, and what appears to be a type of Bob-omb.
 * Helicopter
 * Mole Tank
 * Submarine
 * Train*
 * Car*