Kinoko Block Yoshi

Kinoko Block Yoshi is an original Yoshi transformation that only appears in Super Mario-kun.

While Mario, Luigi, Yoshi, and Toad are fighting the Mask Gate at Wart's Castle, the Mask Gate chomps on Yoshi and, while he is still in its beak, Mario tosses a Mushroom Block at it, 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, author Yukio Sawada states that this would be the last "Kirby-esque" transformation for Yoshi, despite such transformations returning in later volumes.