Autobomb

Autobombs are large, gray, wheeled cannons which launch projectiles and can be ridden on. It is a personal favorite of Shy Guys, and are the evil dreams' assault battery.

Yume Kōjō: Doki Doki Panic
In Yume Kōjō: Doki Doki Panic, 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, riding an Autobomb was mandatory in order to cross large, spike-filled gaps. The Autobomb itself can be destroyed with a thrown object.

Super Mario Bros. 2
In Super Mario Bros. 2, Autobombs are still manned by red Shy Guys and act the same way they do in Yume Kōjō: Doki Doki Panic. In the NES version, they are black, white and red, in the SNES version, they are teal, and in Game Boy Advance version, they are gray.

"Koopenstein"
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.

Trivia

 * The "M" emblazoned on the side of every Autobomb may stand for Mamu, the Japanese name of Wart, or Muu, the original name of Subcon.
 * In the manual to Super Mario Bros 2 on the NES, it's stated that the Autobomb is sometimes referred to as a "bad dream machine".