Bullet Bill

Bullet bills have been around since the very beginning. The are fired from canons and either go in a straight line or home in on Mario (depending on the game). They are one of the coolest looking early Mario enemies and also add an edge to the game (an enemy used to get across gaps), which is later used by other game developers. My favorite uses of the bullet bill include Super Mario Sunshine, where you manipulate them into blowing stuff up for you when they chase you, and both Kaizo Mario World, a hacked version of Super Mario World where you use them to get across the stage, and IWBTG, where you do the same thing.