The Metroid was created long ago by an ancient tribe known as "The Chozo" in order to fend off a threat known as the X Parasite. The Metroids were discovered on Planet SR388 years after their creation. After the Space Pirates discovered the Metroids, they were promptly used as bioweapons, due to the Energy Leeching abilities the creatures possess.
In the original Metroid, Samus was sent to Planet Zebes to destroy the Space Pirates, as well as dispose the stolen Metroids. In Metroid II: Return of Samus, the Galactic Federation noted the danger the Metroid posed, and called upon Samus to go to SR388 to eradicate the Metroids once and for all. During the last battle with the Queen Metroid, an infant Metroid hatched and imprinted Samus as her mother. This last Metroid became known as "The Hatchling" and would become one of the most important Metroid in the series.
In Super Smash Flash 2
It returns as an Assist Trophy from Super Smash Bros. Brawl, and as such, it carries over the same function. After being released, it will fly around and eventually latch onto the head of one of the opponents. Once latched, the Metroid will start draining its Life Energy, racking up the damage meter of the opponent. Unlike in Brawl, movement is not hampered when a Metroid latches onto a character, as the character can move about freely. The Metroid can also be attacked, which can cause it to let go of the opponent it's latched onto.
It was said that the Metroid in Super Smash Flash 2 was actually the Metroid Hatchling, supported by its small size compared to how it originally appeared in Super Smash Bros. Brawl. However, this was proven to be false when it was enlarged in size to make it more similar to regular the Metroid in Brawl.
Metroid, Mother Brain and Starfy are the only Assist characters that the playable fighters can attack.