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 basic enemies attached to and holding onto balloons, are rare and only appear in one level.
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.