The Boomerang Bros. are a Koopa species cousin to the Hammer Bros. which use boomerangs instead of hammers.
Super Mario Bros. 3
The boomerang bros. first game appearance was in Super Mario Bros. 3. They are a common enemy type in Desert Land. They are defeated by jumping on them or throwing a fireball or shell. In Dark Land the bros. appear as captains of Bowser's Tank and forces.
New Super Mario bros.
In this game the Boomerang Bros. rarely appear, but are in World 2, 7, and 8.
New Super Mario Bros Wii
In this sequel the Boomerang Bros. appear again, but only in world 2.
Super Mario Galaxy 2
Super Mario Galaxy 2 marks the first 3D platformer for the Booomerang Bros. Yoshi can catch the boomerangs and spit them back at the bros.