Big Metroid



The Big 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, its 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. For reasons currently unknown, the Metroid was not only restricted to its larval stage, but it had also 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. This mutation was never witnessed within the Metroid specimens featured in Metroid and its remake, Zero Mission. Currently, this larval Metroid anomaly's only known occurrence was with the Infant Metroid stolen by the Space Pirates from the Ceres Space Station and brought to Zebes. 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 damage 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 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. However, since it is implied in Other M that the Baby had the potential to become a Queen Metroid, and Queens follow a separate metamorphic path to other Metroids, it is possible that the Big Metroid was a stage in the Baby's evolution into a Queen.



Super Metroid Nintendo Player's Guide
"There's nothing you can do to get it off of you."

Trivia

 * Matt Manchester, an environment artist for Metroid Prime 3: Corruption, wrote design notes explaining why the Metroid increases in size, but they are not considered canon as they did not make it into the final game due to text size limitations: "Observations of captive Metroids display a rare anomaly in their developmental growth. Approximately .08% of contained specimens never reach the biological process that leads to their otherwise natural metamorphic stages of maturity. While developmental stages are stunted, physical growth continues at a congruent rate. Efficiency of energy consumption increases steadily as well. During behavioral analysis, greater intelligence and problem solving skills are observed. Heightened abilities of energy consumption make this anomaly a potential key in isolating and duplicating their feeding process artificially."

In his notes, he considered placing a Big Metroid in a containment in Metroid Processing, though this never came to fruition.
 * Since the Baby was the only existing Metroid during Super Metroid's events, it would be logical to believe 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 first mission on Zebes did not turn into Big Metroids after exposure to beta rays.
 * Interestingly enough, the Omega Metroid seen at the end of Metroid Fusion also suffered from an unnatural growth. Since this Metroid was also cloned from the Baby like the Queen Metroid in Other M, it may also have been transforming into a Queen. However, this was never confirmed.