The Metroid (メトロイド Metoroido) is a species of extraterrestrial creature from the Metroid universe. Metroids are parasitic life forms that suck the life out of other living things to sustain themselves. This makes them a constant target for Samus' arch enemies, the Space Pirates. Samus eventually puts them into extinction, which causes their prey, the X-Parasites, to come back from suppression and attack Samus, so she has to get a Metroid vaccination in Metroid Fusion.
The Metroid appears in Melee as a collectible trophy, unlocked as one of the 100+ trophies that can be collected randomly during normal play, such as in the Trophy Lottery and throughout the various Single-player Regular Matches. Its description reads as follows:
A parasitic life form that can absorb all types of energy, Metroids have strong resistance to most conventional weaponry. To defeat them, Samus Aran had to freeze them with Ice Beam shots and then blast them with missiles. Mochtroids, which look like Metroids, are weaker, with fewer internal nuclei. (Metroid 08/89).
 As an Assist Trophy
Metroid is featured in Brawl as an Assist Trophy. When a Metroid is released, it will attempt to latch onto a character's head, slightly immobilize them, and sap their health. It can be shaken off by moving about rapidly (or with a simple air dodge), upon which it may seek another victim. It moves in a rather inertial way (like Super Sonic and Volt Tackle), floating about the stage lazily. It can also pass through walls. The Metroid does damage much like a faster version of the flower effect, except that it makes characters flinch - this makes it very difficult to accomplish anything while being sucked, and it causes CPU opponents to use their up special move repeatedly while attempting to recover. However, recovery moves like Spin Attack if used repeatedly while a Metroid is latched, the player is able to go very high, and possibly get past the upper blast line. Like Starfy and Lakitu, Metroid can be attacked, but it will only flinch away a little bit and then attempts to find a victim again. It does not harm the summoner.
 Assist Trophy Origin
Metroids first appeared in Metroid as an enemy and played a crucial part for the games storyline, Metroids have also appeared in every Metroid game so far. Metroids are a species that live on SR388, they live by absorbing energy from other organisms with their gigantic teeth, when they attack Samus Aran the Metroid will attach onto Samus and drain her energy from her suit, the only way to make a Metroid detach from Samus is to drop a morph ball bomb which will hurt the Metroid and release Samus. In Brawl, the Metroids act the same as they do in the Metroid games as they grab on to a player to cause damage. They can also phase through platforms, similar to a Phazon Metroid in Metroid Prime 3: Corruption.
 Trophy Info
A bioengineered life-form found on planet SR388. Metroids attach to organisms and drain life energy. The Galactic Federation commissions 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.