Bouncing Bullet Bill
From the Super Mario Wiki
Bouncing Bullet Bills are a type of yellow Bullet Bill that appears in Super Mario World 2: Yoshi's Island and its Game Boy Advance remake, Yoshi's Island: Super Mario Advance 3. Fired from unique swiveling yellow Bill Blasters, they have the ability to bounce off of any surface that they come in contact with. They only appear in Raphael the Raven's castle and Fight Toadies w/ Toadies. Like other Bullet Bills found in Super Mario World 2: Yoshi's Island, they can be defeated with a jump or egg. They can also be eaten by Yoshi and spat back out to damage other enemies, or swallowed to produce a Yoshi Egg. After being spat out, they behave like a regular Bullet Bill and don't bounce off walls any more.