Hammer Bros. (also called Hammer Brothers; called Hammer Bros in many games) are a species of Koopa that stand upright and throw hammers at the player. They first appeared in the game Happy Tree Friends Adventures. They have bigger shells than normal Koopas and wear helmets, most likely to protect themselves from their hammers. 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 Troop.