Metroids are an enemy from the Metroid series of games. They make a cameo in Kirby's Dream Land 3 along with Samus Aran (also from the Metroid games). Metroids are biologically engineered parasitic life forms that latch onto and drain the life energy of a victim through an unknown process.
Kirby finds six Metroids in the second stage of Iceberg. There, they'll erratically fly around and attempt to latch onto Kirby. Oddly, if one does manage to get a hold of Kirby, the Metroid is unable to do any damage to him. To get the Heart Star for this stage, Kirby must defeat all Metroids; because their principal weakness in the Metroid series has always been extreme cold (usually presented in the form of Samus's "Ice Beam" weapon), Kirby must get the Ice ability and freeze the Metroids. At the end of the stage, if all the Metroids are wiped out, Samus will appear before Kirby without her helmet on to signify the mission was successful, a likely reference to the typical Metroid convention of allowing players to get a glimpse of Samus's face (and usually body) by attaining a 100% collection rating. In exchange for his help Samus will gratefully give Kirby her Heart Star.
In the Super Smash Bros. series
Metroids appear in the game Super Smash Bros. Brawl as an Assist Trophy and collectible trophy. As the Assist Trophy, Metroids erratically fly around the stage. They'll emit a screech every now and then before aggressively charging at a player and latching onto him/her. From that point, the Metroid will damage the player. Soon after, the Metroid will get off of that player and resume its random flight around the stage, soon latching onto another (or in some cases, the same) player. The Metroid will do this a few more times before vanishing in a plume of smoke.
Metroids return as an Assist Trophy in Super Smash Bros. for Nintendo 3DS/Wii U, acting exactly the same as they did in Super Smash Bros. Brawl. They also appear as a collectible trophy in the games.
A bioengineered life-form found on planet SR388. Metroids attach to organisms and drain life energy. The Galactic Federation comissions Samus to eliminate them, but Space Pirates try to harness their power. One of the few ways Samus can kill Metroids is by shooting them with the Ice Beam and then shattering them with missiles. The Metroid's cry is chilling and indescribable.