Bouncing Bullet Bill
From the Super Mario Wiki
In their original appearances, these yellow Bullet Bills are fired from unique swiveling yellow Bill Blasters, and 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 an 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.
In Yoshi's New Island, their color was changed from yellow to green.