Boom Boom

Boom Boom is a henchman of Bowser that Mario fights in each fortress in Super Mario Bros. 3. He is very easy to defeat; all Mario has to do is 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. Boom Boom would begin by simply walking left and right towards Mario. After being hit, he would start to charge, jump, and fly.

Whether or not 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 Super Mario Advance 4 manual refers to Boom Boom as singular in the desription: "Boom Boom, the fateful servant of Bowser."

Cartoons
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.

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

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.