Balloon Boo

Not to be confused with Boo Balloon

Balloon Boos are large relatives to Big Boos. They are small, and act like normal Boos, but if looked at, they will start sucking in air. The more air they suck in, the larger they get. If you turn your back to them, they will start to chase you, letting out the air they sucked in in the process, and thus decreasing in size again.

Balloon Boos only appear in New Super Mario Bros. twice, both in Ghost Houses.