Spike Ball is an ability learned by Bowser in the game Mario & Luigi: Bowser's Inside Story. The ability is learned when Mario and Luigi cure Bowser of his back pain by drilling into a spot at the Lumbar Nook, however this also causes Bowser to become more flexible and allows him to use the Spike Ball ability. This ability allows Bowser to curl up in his shell and climb walls of soft dirt, smash through certain objects, get shot through a spring and activate treadmills in the overworld.
When Bowser learns the Spike Ball, he suggests that he could never roll up into a ball before. However, the Special Attack Shy Guy Squad has Bowser roll up into a ball. These Shy Guys are rescued long before Bowser learns the Spike Ball.
A similar ability was utilized in Super Mario Galaxy during the various fights against Bowser.