Boom Boom

Boom Boom (sometimes stylized as BOOM-BOOM) is a henchman of Bowser that Mario fights in each fortress in Super Mario Bros. 3, before the player fights a Koopaling in an airship.

Super Mario Bros. 3
In Super Mario Bros. 3, to defeat Boom Boom, Mario has to jump on his head three times to eliminate him, or hit him with fireballs or hammers if he possessed the Fire Mario or Hammer Mario power-ups, respectively. Boom Boom can also temporarily crouch down and reveal his spikes, which, if jumped on, would result in Mario powering down or losing a life. Boom Boom would begin by simply walking left and right towards Mario. After being hit, he would start to charge, jump, and in some cases, fly. After being defeated, he will release a ? Ball, which destroys the fortress Boom Boom was in.

Whether Boom Boom is a true species or just a recurring single character is unclear, but there are at least two Boom Booms in existence, as a few extra levels in Super Mario Advance 4: Super Mario Bros. 3 will pit two Boom Booms against Mario simultaneously. However, Boom Boom may be only one character, with clones of himself. This is possible because the manual refers to Boom Boom as singular in the description: "Boom Boom, the faithful servant of Bowser."

The Adventures of Super Mario Bros. 3
At least one Boom Boom appeared in Mario cartoon series The Adventures of Super Mario Bros. 3, where he was nothing more than a silent power house. Much like in the game, only one Boom Boom was ever seen at any given time.

In the Super Mario Bros. film, there was a bar called the "Boom Boom Bar," which was featured with some prominence in the movie.

In the book Koopa Capers from the Nintendo Adventure Books series, there is an item called Boom Boom's Socks, which is a reference to Boom Boom.

Trivia

 * In Super Paper Mario, one of the Sammer Guys that can be fought is named Flailing Boom Boom, which is an apparent reference to Boom Boom and how he would flail his arms around in battle.
 * Boom Boom has been announced to be in Super Mario 3DS.