Boo Balloon

A Boo Balloon is a Boo in the shape of a blue balloon in Super Mario World 2: Yoshi's Island. They race about the screen, trying to deal damage to Yoshi. They will inflate when hit with an egg. After three hits, they will explode, yielding coins or sometimes a ! Switch. They appear in Bigger Boo's Fort and Don't Look Back!, with the former called Blue Boos in the Nintendo Power SNES guide due to the black background rendering the string invisible.

Super Mario World 2: Yoshi's Island

 *  Official Nintendo Player's Guide bios
 * Blue Boo: They only flutter in the darkest chamber of Bigger Boo's castle. Hit them with three eggs.
 * Blue Balloon: If he could, Boo Balloon would get a mowhawk and a bunch of tattoos to show what a toughie he is.