They have bigger shells than Koopa Troopas and wear helmets, usually of the shell's color. Hammer Bros (also known as Hammer Brothers and sometimes formatted with a full stop in older material) are Koopas that stand upright and throw hammers at the player, first appearing in Super Mario Bros.
“Our hammers fly high! Our hammers fly fast! And sometimes our hammers even hit something!” - Hammer Bros, Mario & Luigi: Paper Jam
Artwork of a Hammer Bro from New Super Mario Bros.