From the Super Mario Wiki
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Metroids are the titular species of parasitic aliens from the Metroid series. They were created by the Chozo to combat the X Parasites, though they eventually grew out of hand, and are now supposedly extinct thanks to the efforts of Samus Aran.
A Metroid appears in the Metroid-Samus microgame in WarioWare: Twisted!. The Metroid advances toward Samus in a different pattern depending on the difficulty level, and the objective is to shoot it with her arm cannon before it can reach her.
Metroids also appear in WarioWare: Touched!, in the international version-exclusive microgame Metroid, and in the Big Name Game also titled Metroid. In the former the objective is to drag Metroids off of Samus using the stylus, and in the latter the objective is to destroy two Metroids by tapping them.
Super Smash Bros. series
A Metroid appears as an Assist Trophy in both Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U; they also appear as enemies in Smash Run in Super Smash Bros. for Nintendo 3DS. When they appear, they float around the stage, latching on to players and inflicting damage for a time, similar to their series of origin.
Super Smash Bros. Melee
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS / Wii U