Slime

A Slime is a gooey creature made of slime, first appearing in Super Mario World 2: Yoshi's Island and its enhanced port Yoshi's Island: Super Mario Advance 3. A notable Slime is Salvo the Slime, who appears in Salvo The Slime's Castle, and marks the first appearance of Slimes in general.

Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Generic Slimes appear in Tap-Tap The Red Nose's Fort and King Bowser's Castle, carrying keys. They are disguised as rectangular platforms until a Yoshi touches them, which makes them morph into their true form. Like Salvo the Slime (who was actually a Lemon Drop before Kamek's transformation), they cannot directly harm Yoshi and Baby Mario, but they push them aside. If a Slime is hit by Watermelon seeds or an egg, it gets progressively smaller until it disappears. The same effect happens to the Slime if it falls into a spike or lava pit.

Yoshi's New Island
In Yoshi's New Island, a single Slime appears in a secret room in Spray Day Mayday. The room can be reached via a door that will spawn by hitting a hidden Winged Cloud above a large block resembling a disguised Slime. The room contains another such block and a few Coins next to it; if Yoshi collects some of these Coins, the block beside will transform into a Slime and start attacking Yoshi as in its original appearances. The Slime can cause Yoshi to lose a life if it pushes him onto the spikes on the left. Defeating the Slime will cause a round of Coins (with several Red Coins under disguise) and a Smiley Flower to appear.

Trivia

 * The Super Mario World 2: Yoshi's Island Player's Guide considers this enemy to be Salvo the Slime. This is changed in the Yoshi's Island: Super Mario Advance 3 Player's Guide.