The Big Metroid, also known as the Super Metroid, was an anomaly in the Metroid life cycle seen in Super Metroid and briefly in Other M. Normally, when a Metroid is not located in the environment of its homeworld, SR388, and is instead inserted on another planet, the different atmosphere and climate seemingly halts the Metroid's natural life cycle and the creature is never able to mutate into the Alpha stage.
However, a never-before-seen mutation occurred with Samus Aran's baby (the last surviving Metroid from SR388's extermination) when it was brought to the planet Zebes. Stolen by the Space Pirates from the Ceres Space Station during a raid led by Ridley, the Metroid was not only restricted to its larval stage, it had also, for reasons unknown, increased dramatically in size. Its leeching attack was far greater than that of a typical Metroid, and it had somehow removed its vulnerability to cold temperatures (an invulnerability seen previously in several Phazon-induced Metroid mutations, and Unfreezable Metroids later on). This mutation was never witnessed within other Metroid specimens on Zebes featured in Metroid, its remake Metroid: Zero Mission and earlier Metroids encountered in Super Metroid.
This giant Metroid is practically immune to all of Samus' weaponry, including the Ice Beam and Power Bombs. Only Mother Brain had the strength to harm it and ultimately kill it; while the Pirate leader was capable of killing it with one of its standard attacks in Super, Other M 's interpretation revealed that she had to rely exclusively on her Laser Brain Attack to destroy it.
Interestingly, the Big Metroid was capable of performing an ability that no other Metroid had previously shown: the ability to transfer energy from one being to another. This life-saving power, originally hinted at by the Ceres scientists' research as possibly being beneficial to Galactic society in Super Metroid 's introduction sequence, allowed Samus to emerge victorious in her battle with Mother Brain.
The reason behind this mutation is never explained in-game. In the Super Metroid comic, Armstrong Houston suggests that the creature was heavily exposed to Beta-Rays which resulted in its unnatural size increase; since the baby was the only existing Metroid during Super Metroid 's events, it is probable that the Space Pirates were forced to use an excessive amount of beta radiation to produce a satisfying amount of Metroids, thus mutating it. This would also explain why the Metroids featured in Samus's Zero Mission did not turn into Big Metroids despite exposure to beta rays. However, since it is stated in Other M that only special Metroids have the specific genetic coding to become Queen Metroids and the Queen featured in the same game was created from the baby's DNA, it is possible that the Big Metroid was a stage in the baby's evolution into a Queen.
The battle against the Big Metroid is scripted in the game. As mentioned before, it is invulnerable to all of Samus' weapons and it is programmed to chase her at an incredible speed and weigh her down as it drains energy off of her, preventing her from leaving the room where it is "fought". It cannot kill Samus, since it is also programmed to stop exactly when one unit of energy remains, triggering the cutscene where the Big Metroid recognizes Samus. However, it is nevertheless possible to "defeat" this creature: Samus can successfully evade the Big Metroid's persistent chase and leave the room using quick maneuvering of her jumping abilities without triggering the mentioned cutscene (a popular tactic among speedrunners). However, if the Metroid is latched onto Samus as she exits the room, she will be glitched in the form of incredible slowdown in her movements, exactly as if the creature was still weighing her down. Another glitch that can result from having the Big Metroid latch onto Samus before leaving its room is the heroine being frozen in her exhausted animation (hit by the Laser Brain Attack and her near-complete energy absorption), unable to recover. Therefore, these glitches force players to r