From the Super Mario Wiki
Some Boos and a Balloon Boo.
Balloon Boos are enemies that appear only in New Super Mario Bros., where they are encountered in World 4-. They are Boos with pink cheeks, and their tail resembles that of a balloon, akin to their name.
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.
- When Mega Mario or Mega Luigi collides with a Balloon Boo, they pop and soar away in circles, somewhat reminiscent to knocking out a Goal Pole.
- Ballon Boo bares a slight resemblance to Fwoosh.