From the Super Mario Wiki
Balloon Goombas are, as their name implies, Goombas attached to floating balloons. They first appear in New Super Mario Bros. U. They are very similar to Balloon Waddle Dees in that they are both each appropriate game series' most basic enemies attached and holding onto balloons and that they both are fairly rare and appear in levels involving beanstalks.
New Super Mario Bros. U
Balloon Goombas' only appearance is in the Sparkling Waters level Skyward Stalk. They float straight upwards from the bottom of the screen on a balloon. When its balloon pops, it will begin walking like a normal Goomba. Mario can jump off the balloons for extra height. They are rare, and there is a total of 6 Balloon Goombas in the game.