Hammer Bros., also pronounced Hammer Brothers and stylized as Hammer Bro. for individual unita, are a species of Koopa that stand upright and throw hammers at passing by-standers. They have been around since Super Mario Bros. They have bigger shells than normal Koopas and wear headgear. They can also use special hammers to shrink their enemies. Hammer Bros. have many times been described as the most elite fighters in Bowser's Koopa army. These enemies were originally designed by Kazuaki Morita.
A Hammer Bro. was originally considered to be a hazard enemy in the Mushroom Kingdom IIIstage from Super Smash Flash 2. As its original behaviour from its source Mario games and the Assist Trophy in Brawl, it was supposed to launch hammers at the brawling players, it would have appeared on stage at certain intervals to later retreat. It was never added into the game and was only hinted - alongside other stuff that never made it into the game like ? blocks or jump blocks - at the DOJO!!!'s page for Mushroom Kingdom III before said DOJO!!! incarnation was scrapped. Additionally, the stage got revamped to work completely different from its original layout. Hammer Bro. officially made an appearance in Beta version, serving as an Assist Trophy.