From the Super Mario Wiki
Autobombs are large, gray, wheeled cannons which launch projectiles and can be ridden on.
Super Mario series
Super Mario Bros. 2
In Super Mario Bros. 2, Autobombs were utilized by red Shy Guys as a means of transportation and offense. They would pursue and shoot fiery projectiles at the player. It can travel down steps but not up. If the Shy Guy was removed, the Autobomb will stop shooting and travel in a straight line. The player can then ride it, but not control it or pick it up. In some levels of Super Mario Bros. 2, riding an Autobomb was mandatory in order to cross large, spike-filled gaps. The Autobomb itself can be destroyed with a thrown object.
The Super Mario Bros. Super Show!
In The Super Mario Bros. Super Show!, Autobombs, never referred to by name, only made one appearance, in the episode "Koopenstein"; an Autobomb first appeared in the episode when Mario, Luigi and Princess Toadstool are captured by Mouser and a group of Shy Guys. Later, Toad, when surrounded by Shy Guys, manages to light the fuse of a nearby Autobomb using a Piranha Plant, causing it to blast the surrounding Shy Guys, creating a distraction that allowed Toad to escape.