Non-canon warning: This article or section contains information that may not be considered an official part of the Metroid series in the overall storyline by Nintendo.
A gatling gun appears in the first chapter of Metroid Prime: Episode of Aether. The Space Pirates, led by a boss take over the Crest ship, and hold the crew and passengers hostage. Samus Aran receives the distress call and rushes to the ship. The Pirates, believing they far outnumber Samus, all fire on her with their weapons, but she is able to miraculously avoid all of it. She picks up this gatling gun from the ground and fires it at her enemies in tandem with her Arm Cannon, much to their shock.
The presence of a gatling gun, a World War II invention that has been replaced by machine guns, in the futuristic Metroid universe is curious to say the least. It is also worth noting that this is the only non-Beam weapon Samus has ever used in the Metroid series.
Non-canon warning: Non-canonical information ends here.