Kinoko Block Yoshi

Kinoko Block Yoshi (キノコブロックヨッシー, Kinoko Burokku Yosshī, Mushroom Block Yoshi) is an original Yoshi transformation that only appears in Super Mario-kun.

Mario, Luigi, Yoshi and Toad are fighting the Mask Gate at Wart's Castle. The Mask chomps on Yoshi and, while he is still in the beak of the enemy, Mario tosses a Mushroom Block at the Mask Gate, but it lands in Yoshi's mouth, turning him into Kinoko Block Yoshi, blocking the Mask Gate's beak, allowing them to access the throne room.

On a side note, the author tells that this would be the last "Kirby-esque" transformation for Yoshi, a promise that will not last since such transformations will be back in later volumes.