Autobomb

Autobombs are large, gray, wheeled cannons which launch projectiles and can be ridden on. It is a personal favorite of Shyguys.

Yume Kōjō: Doki Doki Panic
In Yume Kōjō: Doki Doki Panic, Autobombs were utilized by red Shyguys 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 Shyguy 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 Shyguys 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 Super Mario All-Stars version, they are teal, and in Super Mario Advance, 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 Shyguys. Later, Toad, when surrounded by Shyguys, manages to light the fuse of a nearby Autobomb using a Piranha Plant, causing it to blast the surrounding Shyguys, creating a distraction that allowed Toad to escape.

Super Mario Bros. 2

 * Instruction Booklet bio: Shyguy's favorite ride. It is also referred to as a "bad dream machine."

Super Mario Advance

 * Instruction Booklet bio: These cannons-on-wheels can be very helpful. Steal them away and use them to cross areas with spiked floors.

Trivia

 * In the original instruction booklet, the "M" emblazoned on the side is red and a different shape on the sprite, whereas it is white in-game. While the artwork is redrawn in later releases, it is still depicted as red instead of white. Said "M" may also stand for Mamu, the Japanese name of Wart, Muu, the original name of Subcon, or possibly Mucho, the Japanese name of Snifit.
 * The Yume Kōjō: Doki Doki Panic instruction booklet states that the Autobomb shoots the shells of bad dreams, and that because of that, it is also known as a terrifying stroller. This may be a remnant of earlier versions of the game where Autobombs shot Bullet Bill-like rounds instead of fireballs, which can be seen in pre-release material. The Super Mario Bros. 2 instruction booklet removes mention of projectiles and instead states that it is also known as a "bad dream machine."