Metroid (species)

Metroids are one of the major elements and creatures in the Metroid series of games. They are some of the more feared species in the game, as they often appear in dark stages and attach themselves onto your face to suck out your energy. The species were created by the Chozo, and named them Metroid apparently because the name means "ultimate warrior" in their language. They created them so that they could be protected by from the X Parasite, and they apparently don't eat blood as some may conclude, but rather suck out their victim's life energy. This allows them to kill the parasites without dying.

However, their project went awry when the Metroids started to become more vicious. No one knows what ever happened to the Chozo on the planet that created them, though it could be conclusive that they were killed by the Metroids. Once a Metroid is born, it considers the first being it sees to be its mother, much like most real life animals do. This can be seen in Metroid II and Super Metroid, where a newly hatched baby Metroid looks to Samus Aran, the protagonist of the sieres, as its mother, as normally a newborn would attack her or any other living being. It can thus be concluded that they have a form of intelligence seeing that they have a thought process.

In time, a Metroid will start to morph (the life cylce of a Metroid can be seen to the left) into larger and more powerful creatures. All of the species to the left have been seen as enemies in the Metroid series.

Queen Metroid
In all, only one Queen Metroid has been found (in the SNES video game Super Metroid). It seems to be the largest of the Metroid species found in the life cycle above, and currently it is unknown what makes a Metroid turn into a Queen Metroid. The Queen Metroid found in Super Metroid is a very strong creature that is capable of tearing its opponents apart easily using its long, expandable neck to grasp onto far away prey.

Metroid Prime (creature)
Metroid Prime is a creature and final boss in the GameCube game of the same name, Metroid Prime. Apparently, according to the official website atleast (which could arguably be considered cannon), Metroid Prime came from the same planet that the Metroid species came from. It was also revealed via the Chozo lores that can be found in the series that Metroid Prime is one of the major reasons why their species started to become extinct.

It should be noted that Prime arrived on Tallon IV long before the events that took place in Metroid Prime. After being defeated by the hands of Samus Aran, the creature was able to regenerate itself and turn into the being known as Dark Samus, who would permanently die in the Wii video game Metroid Prime 3: Corruption.

Involvement with Samus Aran
Metroids play a large role in Samus' life. In the first chronological story, which is also the first game in the series, Samus was ordered by the Galactic Federation to hault the Space Pirate's Metroid production. In all games Samus is often times forced to defeat the Metroids that are attacking her.

As aforementioned, a newborn Metroid (in Metroid II and Super Metroid) assumed that Samus Aran was its mother, and at one point even saved her from a raging monster once it matured into an older creature.

In the games, if a Metroid starts to suck Samus' life energy out of her, she'll have to go into Morph Ball mode and bomb the creature off of her, though that doesn't entirely mean it will die. Usually Metroids are hard to defeat, taking multiple shots, sometimes even many missiles to ultimately kill. In Metroid Prime 3: Corruption, they could easily be defeated when in hyper mode. In side-scrollers, they could only be killed once frozen, where Samus will then have to shoot the Metroid which is trapped in ice.