Balloon Boo

Balloon Boos are enemies that have only appeared in New Super Mario Bros., where they are encountered in. They are Boos with pink cheeks, and their tail resembles that of a balloon, akin to their name. Only five appeared in the whole game.

They appear large at first, but when looked away from, they approach to the player while expelling air and shrinking. Otherwise, they stop and inflate by inhaling air at a rather slow rate, until it has assumed its massive appearance once more. Balloon Boos will continue to chase Mario or Luigi even when they have reached their minimum size; they will also only chase them if they are in the same room (i.e. there are no walls between them). Balloon Boos can be defeated with a Blue Shell or a Mega Mushroom.

Trivia

 * When Mega Mario (or Mega Luigi) comes into contact with a Balloon Boo, it flies away in circles, somewhat similar to a balloon being let go.
 * The Balloon Boo has a small and uncanny resemblance to Kirby, with its small red cheeks and ability to inhale.