The Metroid (メトロイド, Metroid) is a species of extraterrestrial creature and the namesake creature of the Metroid universe. Metroids are parasitic life forms that suck the life energy out of other living things to sustain themselves. This makes them a constant target for Samus' archenemies, the Space Pirates. Samus eventually put them into extinction, which caused their prey, the X Parasites, to multiply and thrive once more. After being infected by an X Parasite, the Galactic Federation saved her life by injecting her with the Vaccine "Metroid", which was made from the Metroid Hatchling from SR388. In addition to saving her life, this also gave her body Metroid-like characteristics, such as immunity to X Parasites and weakness to cold temperatures. During her mission at the infested Biologic Space Labs Research Station, she discovered that the Galactic Federation had begun a Metroid breeding program. Upon this discovery, she set the ship's path to SR388, destroying the last remaining Metroids upon impact.
Metroids first appeared in Metroid as an enemy and played a crucial part in the game's storyline. Metroids have also appeared in every Metroid game since except for Metroid Prime Hunters. Metroids are a species that live on SR388, and survive by latching onto other organisms and draining their energy with their gigantic teeth. Their greatest weakness is cold, so Samus can destroy them with her Ice Beam or Ice Missiles. The only way for Samus to shake off a Metroid is to drop a bomb while in her Morph Ball form, which deals damage and detaches it from her. In Brawl and SSB4, Metroids act the same as they do in the Metroid series, grabbing players and dealing damage. They can also phase through platforms, similar to a Phazon Metroid in Metroid Prime 3: Corruption.
The form most commonly depicted in the Super Smash Bros. series is the Metroid larva, although other forms have made appearances in the series. The fully-matured Queen Metroid (using its appearance from Metroid: Other M) and Phazon-mutated Metroid Prime have appeared as trophies; a Tallon Metroid appears as a sticker; and Dark Samus (a reincarnated form of the Metroid Prime) is a fully playable character in Ultimate.
The Metroid was mentioned in Samus's biography.
The Metroid appears in Melee as a collectible trophy, unlocked as one of the 200+ trophies that can be collected randomly during normal play, such as in the Trophy Lottery and throughout the various Single-player Regular Matches.
As an Assist Trophy
Metroid is featured in Brawl as an Assist Trophy. When a Metroid is released, it will attempt to latch onto an opponent'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 - only a few attacks can damage, ones that attack on frame 1 - 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, if recovery moves like Spin Attack are used repeatedly while a Metroid is latched, they can cause a high amount of vertical movement, possibly even pushing the character past the upper blast line. Like Starfy and Lakitu, Metroid can be attacked, but it will only flinch away a little bit and then attempt to find a victim again. If the victim is KO'd, the Metroid will be KO'd as well.
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.
As an Assist Trophy
The Metroid returns in both versions of the game as an Assist Trophy, functioning the same as in Brawl.
In Smash Tour
A Metroid can appear as a random event. It wanders around the board and steals stat boosts from the first player that touches it: whoever catches it can then retrieve the stolen stats. Retrieving one's own stolen stats is one of the Challenges.
As an Assist Trophy
The Metroid returns once again as an Assist Trophy, functioning the same as in the previous installments. However, it can now be defeated with freezing attacks, like in Smash Run. Training Mode describes its Assist Trophy: Latches on to an opponent and steals their energy. It can be killed if frozen.