Balloon Boo

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 the player turns away from them, they will start to chase the player, letting out the air they sucked in in the process, and thus decreasing in size again. They can't pass through walls, and will not notice Mario unless he appears in their area.

Balloon Boos only appear in New Super Mario Bros., only in World 4-Ghost House. There was only five found in that level.