Balloon Boo

Balloon Boos are large relatives to Big Boos that appear in New Super Mario Bros., only in World 4-Ghost House. 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. Balloon Boos can be knocked out with a Blue Shell or a Mega Mushroom. They won't notice Mario unless he appears in their area.

Trivia

 * Balloon Boos have a similarity to Kirby, as both can inflate to great sizes.