A capsule (カプセル) is a type of device in the Mega Man franchise that is mainly used to keep a Reploid inside for maintenance or other purposes. Some teleportation devices are also shaped like capsules.
The Journal of Dr. Cain mentions that Dr. Cain found X inside a 14 meters high and 8 meters wide capsule in the ruins from Dr. Light's Laboratory. Dr. Light sealed X inside the capsule to check the reliability of his systems for 30 years.
The capsule is not seen in the original game, but appears in the remake Mega Man Maverick Hunter X and in other media. In the remake it is shown to have housed X while Dr. Light built him. In the Mega Man comic book series by Archie Comics, X's capsule has the same appearance as the Light Capsules.
Mega Man X capsule
In Sigma's hideout, Vile locked Zero inside a capsule that has Sigma's mark on the lower part. After X is captured, Zero manages to break free and jumps on Vile, detonating himself to take the Maverick with him, but only destroyed Vile's Ride Armor.
Mega Man X4 capsules
Mega Man X5 capsules
Mega Man X6 capsule
In Zero's ending, he is sealed inside a capsule to be analyzed.
Mega Man ZX Advent capsules
The game's gacha is represented as a capsule, having a design similar to the one in Mega Man ZX Advent.
This capsule also appeared in the Rockman X DiVE collaboration with Tower of Saviors, the event gacha being named Atom Capsule (原子膠囊).