Balloon Boos are big Boos with pink cheeks.
Balloon Boos act like normal Boos, but a little bit different. When you're looking at a Balloon Boo, it will suck in air, making itself bigger. This air won't effect Mario. When you're looking away, the Balloon Boo will chase you, using the air it sucked in. This will make the Balloon Boo shrink, until it's around the size of Mario.