Super Happy Yoshi
From the Super Mario Wiki
Super Happy Yoshi is a special form of Yoshi that appears in Yoshi's Story. It is activated whenever a Yoshi eats a Heart Fruit. Super Happy Yoshi is invincible and stronger and has a longer tongue, an infinite supply of eggs, and a better Flutter Jump. On an additional note, whenever a Black Yoshi or a White Yoshi is in Super Happy Yoshi mode, the Yoshi's mobility while using the Flutter Jump is increased drastically.
If a Yoshi performs a Ground Pound, he will pop all bubbles on-screen, Shy Guys will be turned into the current Lucky Fruit, and peppers will turn into coins. Sometimes when he transforms, some coins will be revealed. While Yoshi is Super Happy, the flower, which represents Yoshi's health, laughs and is completely filled. Super Happy Yoshi's theme music is a rock remix of the current level's music.
If Yoshi transforms as a result of collecting six melons from a mini-game, some coins in the shape of a letter will be revealed. Doing this in all 24 levels of the game spells out a secret message, "YOU ARE YOSHI'S GREAT PARTNER."
Super Happy Yoshi lasts for only a limited time, but its effects will wear off immediately if a Yoshi eats anything like a Black Shy Guy or a pepper or if a Yoshi is eaten by a Piranha Pest. Moving to a new area will also end the effect early. Lastly, the form's invincibility does not protect the Yoshi from fatal hazards, such as Blurps, pits, lava, and compressors.