Capsule

Capsules (カプセル), also known as Metal Cages, or Egg Prisons are large prison units created and used by Doctor Eggman to hold captive animals, presumably so that they can later be turned into badniks. First time seen in original 16-bit version of Sonic the Hedgehog, capsules have appeared in various shapes, but are most often a metal container with a single switch on the top. Pressing the switch will either destroy or open the Capsule, thereby releasing the prisoners.

Description
There are numerous, different capsule designs being featured in the franchise, which are all created by Doctor Eggman to imprison animals friends inside of them. There have been exceptions being including something else in capsules. It is described as large grey unit, that has small support pedestals beneath it. The common aspect seen in Eggman's mechanical creations (aside of badniks), capsules are seen to have yellow and black stripes set on them. On the top of it is usually seen the switch, that opens all loose hatches, while releasing all animals inside.

One capsule is generally featured at the end of single, second or third act of zone / action stage rather having signpost. By pressing the switch on top of it (or hitting the capsule at one time) releases hatches around the capsule, thus releasing animals out of it. The player usually finds it right near boss arena after defeating the boss of zone.

In recent games, starting from Sonic Adventure 2, Egg Capsules have been replaced by Goal Rings, though a variation of the Egg Capsule appears in some levels of Sonic Colors, which Sonic must destroy to save the captured Wisps. Despite that, capsules have been featured in occasional times.

Sonic the Hedgehog (1991)
Capsules are first featured in original 16-bit version of the Sonic the Hedgehog, as being seen at the end of zones's third act, following a boss battle. These capsules are light grey, shapely units that have yellow and black stripes and an small window at the right front. It has four small support pedestals under it. At top of the capsule is grey switch. After pressing the switch, the capsule starts exploding while the player moves automatically after the doctor. After exploding, dozens of small animal friends are freed and quickly moving away in both directions, leaving behind a pile of remnants of the capsule and results starts popping out. Scrap Brain Zone act 3 doesn't feature capsule, as the player is transferred to Final Zone automatically after completing it.

8-bit version of the game with same name also features capsule (called as Metal Cages in american instruction manual) at the end of third act at each zone, which is short and only has boss battle. Capsules are more thinner than in 16-bit counterpart, but does mostly have same method, while the player moves to the next zone and animals run to the other direction. Only third act zones that don't have capsules in the end are Scrap Brain Zone and Sky Base Zone.

Sonic the Hedgehog 2
Capsules is next seen in 16-bit version of Sonic the Hedgehog 2, where a single capsule is now featured at the end of second act on each zone, after defeating one of Robotnik's vehicles. This capsule's design is similar to previous one, but it is more detailed. There are four hatches, black and yellow stripping on lower part and four support pedestals beneath it. There is yellow switch, which by pressing it opens hatches and releases animals inside of it. After doing that, the player is able to move at background while results are being calculated. Capsules are not featured at Sky Chase Zone, Wing Fortress Zone and Death Egg Zone, which only have one act.

Drastically different looking capsules (called as Egg Prisons in american manual) are featured in 8-bit version of the game with same name. These variations float in the air and are round-shaped, resembling Robotnik himself, as having eyes and mustaches. Instead of a switch, the player has to do Spin Jump onto it and the capsule will open its bottom hatch. Sonic moves to the next zone while capsule keeps exploding and all animals fall out the capsule and run away. Crystal Egg Zone don't feature capsules as featuring two different endings after completing boss from either of the zones.

Sonic CD
Capsules are next seen in Sonic CD, at third zone of the round while having slightly different design. These capsules are large units, which have bigger pillar and round pod at its top with black and yellow stripe and two small antennas. The color scheme and shading changes depending the level and its timeline. Instead of animals, these capsules contain seven flower seeds of Little Planet, which are also used to power up badniks in the game. After hitting capsule once after the boss, the player moves to the next round, while capsule is exploding and seven seeds land to the ground and increase flowers to the place. Capsule is not seen at Stardust Speedway (saving Amy Rose from being tied up to the pole) and Metallic Madness (ending the game after final boss).

Sonic Spinball
The Sonic the Hedgehog 2 variation of capsule is seen in Sonic the Hedgehog Spinball, as being part of third bonus stage called "The March". Here, the player has to destroy the capsule and free the animals, while dozens of chicken robots that resemble Scratch are walking around the capsule. The player has to enough of chicken robots to get hit the capsule.

Sonic the Hedgehog 3 & Knuckles
Capsules are seen repeating role in Sonic the Hedgehog 3 and its add-on continuation Sonic & Knuckles. After beating the boss, it is being featured at the end of second act on each zone. There are two variations of capsules. The first, standard one is similar to ones from Sonic the Hedgehog 2, although being more round-shaped. The second variation is similar to first one, but it is upside down and able to fly by using small rotors on support pedestals. Flying capsule variations are seen at Angel Island Zone, Marble Garden Zone, Launch Base Zone (With Sonic 3 alone, the game ends at this point) and Lava Reef Zone (Knuckles is not able to get this due having different route). Capsules are not seen at Hidden Palace Zone, Sky Sanctuary Zone and Death Egg Zone.

The Flying Battery Zone, Eggman's floating fortress contains numerous capsules throughout the zone and at the zone's background. Some do contain animal friends, but others contain rings, badniks, or nothing at all. In some cases, the switch on top of the capsule may act as a spring. Capsule at the end of the Zone's first act will turn into the act's sub-boss Gapsule, which has eyes and two large spiked balls.

Sonic Adventure
Capsules make third dimensional appearance in Sonic Adventure, as these units appear at the end of some of Sonic and Tails' Action Stages. The capsule design is more simple, but bulky. It has yellow and black striping around, small foot supports and large releasing button on the top. Once the capsule is opened, the player has officially reached the end of the action stage and the scores are tallied. These capsules are also seen at action stages that aren't featured at first place, if the player has already beat the game.

Sonic Pocket Adventure
Capsules are normally seen in Sonic the Hedgehog Pocket Adventure, as being featured at the end of second act on each zone after beating the boss vehicle. In the game, capsules have taken the design from Sonic the Hedgehog 2. Capsules are not featured at Sky Chase, Aerobase, Gigantic Angel, Last Utopia and Chaotic Space.

Sonic Advance games
Capsules are featured in Sonic Advance games, although having different design and featured different way at every game. In the first title, it repeats its role from the previous games, as after beating the boss, it is being featured at the end of second act on each zone. Capsule's simple design is hybrid of between Sonic the Hedgehog 2 and Sonic Adventure, but having now Eggman Empire logo on it. It can be found near right by of boss arena, but in some zones, it smashed down to the ground, while the crashing doesn't hurt the player, only bouncing him back. Capsule is not featured at Egg Rocket Zone and Cosmic Angel Zone, which are only one act zones.

Up until Sonic Advance 2, all of the handheld Sonic games have had Egg Capsule at the end of the stage.

In Sonic Advance 3, there's minigame including Egg Capsule with more buttons, which must be pressed to open it and got some 1-ups from minigame.

Sonic Heroes
Sonic Heroes

Sonic Rivals 2
Sonic Rivals 2

Sonic Chronicles: The Dark Brotherhood
In Sonic Chronicles: The Dark Brotherhood, lots of Egg Capsules can be found anywhere of Metropolis.

Sonic Generations
Egg capsules also made an appearance in Sonic Generations in a classic mission on both Seaside Hill and Planet Wisp.

Sonic the Hedgehog Story Comic
Egg Capsule appears in Sonic the Hedgehog Story Comic Volume 2 as doctor Eggman succeeds in capturing the animals of South Island to one of Egg Capsules. After Ricky and Cucky gets freed by Sonic, they lead Sonic to the Star Land Zone where Egg Capsule is located. After beating Orbinaut and Eggman who had Eggmobile with a spike underneath, Sonic press the button on top of Egg Capsule and frees all animals of South Island.

Archie Comics
During the events of the Genesis arc, Sonic defeated Snively Robotnik and freed many forest animals from an Egg Capsule that Snively was guarding, which contained Sally Acorn, Antoine D'Coolette, and Boomer. Sonic then joined the Freedom Fighters in attempt to locate Dr. Robotnik.

Trivia

 * In Sonic the Hedgehog Story Comic Volume 2, "Clash!! South Island", the Egg Capsule is shown to possess retractable robotic arms that allow it to actually capture the animal friends itself.