From the Super Mario Wiki
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The spike ball is an ability learned by Bowser in the DS game, Mario & Luigi: Bowser's Inside Story. The ability is learned at Peach's Castle Garden when Mario and Luigi cure Bowser of his back pain by drilling into the center of the Lumbar Nook. When they finish the process, Bowser's back loosens; he becomes more flexible and can use the spike ball ability. This ability allows Bowser to roll up into his shell to become a spiky ball when the player selects the ability using the shoulder buttons. He can use this ability to climb walls of soft dirt, smash through certain objects, get shot through a spring and activate treadmills in the overworld.
A slightly similar ability was utilized by Bowser in Super Mario Galaxy during the last fight against him in the final galaxy of the game, Bowser's Galaxy Reactor. It could be countered by spinning a Stretch Plant at him during the second phase of the fight.