Mites are tiny paper-thin creatures fought during the events of the Subspace Emmissary. They come in different colors-red, yellow, and green. Though not the strongest of enemies, they are usually in groups. If you're on a side-scrolling level, they can even hit you back far enough so that you fall back and lose a life!
Sometimes Mites appear out of portals, which makes them more annoying as you can't get rid of them completely until the portal is destroyed.
 Smash Run
Mites make an appearance in Smash Run in Super Smash Brothers for 3DS. While they aren't the strongest of enemies, they attack in swarms and are sometimes accompanied by stronger enemies.