Balloon Boo

Balloon Boos are similar relatives to Big Boos that appear only in New Super Mario Bros., encountered in World 4-Ghost House. They are Boos with pink cheeks and their tail resembles a balloon's instead of a ghost's. When ignored, they will persue the player, while expelling air, and reduce in size; allowing the player to take advantage to progress. When observed, they will inflate by inhaling air at a rather slow rate, also increasing in size, until it has become massive. They will proceed to chase Mario or Luigi, even when they have reached their minimum size. Balloon Boos can be defeated with a Blue Shell or a Mega Mushroom. Unlike regular Boos however, they will only react to the player when the Mario Brothers arrive in a certain location containing Balloon Boos.

Trivia

 * When Mega Mario or Mega Luigi collides with a Balloon Boo, they pop and soar away in circles.
 * A similar species in New Super Mario Bros. 2, called the Boohemoth, looks like a Balloon Boo, but does not act like one.