From Sonic Retro
Mecha Sonic refers to a robotic copy of Sonic created by Doctor Eggman (not to be confused with the more well known Metal Sonic). His Sonic the Hedgehog 2 incarnation (in both the 8-bit and 16-bit versions) is referred to as Silver Sonic in the European manual.
Sonic 2 8-bit
The first appearance of Mecha Sonic is the metallic robot that Sonic fights in Scrambled Egg Zone on the 8-bit version of Sonic the Hedgehog 2. Once defeated, Mecha Sonic releases the sixth and final Chaos Emerald if Sonic already has the other 5, enabling progress to Crystal Egg Zone.
This version of the robot is a same-size imitation of Sonic, mimicking all of his abilities, although strangely Mecha Sonic has a spin dash attack and Sonic does not in that game. When Sonic and Mecha Sonic's spin attacks collide, they bounce off each other, unlike the 16-bit iterations where Sonic comes off the loser. This makes for a fast and hectic, if easier, battle. Mecha Sonic is also equipped jet boots that enable him to hover, and a sharp scissors-type tentacle, which sometimes protrudes from its torso when standing still. The robot takes 8 hits to kill.
Sonic 2 16-bit
A silver Mecha Sonic appears in as the first boss of Death Egg Zone on the 16-bit version of Sonic the Hedgehog 2 as well. Sonic walks into the battle arena immediatly after the start of the level, and Mecha Sonic descends using jet shoes - although he doesn't use them to hover during the rest of the fight, unlike the 8-bit version. The boss has four principal attacks:
Eggman watches the battle through a TV screen in the center of the arena, grinning maniacally throughout. Once Mecha Sonic is defeated, Sonic chases the (surprisingly spry) fat man until he leaps into the game's last boss, Dr. Robotnik's Giant Mech.
Sonic & Knuckles
The robot Sonic in Sonic & Knuckles is usually referred to as Mecha Sonic also.
In Sonic & Knuckles, there are four different encounters with Mecha Sonic. They all take place in Sky Sanctuary Zone. He first appears with a ball and chain weapon similar to the one Robotnik uses in the first Sonic the Hedgehog. Later he appears with 8 bubble-like shields similar to a boss battle in Sonic 2 on the third act of Metropolis Zone. The next encounter is similar to the battle with him in the Death Egg Zone in Sonic 2 except for a new backwards dash attack. If playing as Knuckles there is a fourth battle. Mecha Sonic will go super by absorbing power from the Master Emerald. During this battle he dashes back and forth occasionally diving downward or shooting glowing orbs. He occasionally has to recharge his power by landing on the Master Emerald, this leaves him vulnerable to attack.
Sonic Pocket Adventure
Mecha Sonic returned to his 16-bit Sonic the Hedgehog 2 design in Sonic the Hedgehog Pocket Adventure as the boss of the fifth level, Aerobase Zone. It is almost identical to the Death Egg Zone version except it has the added ability to dash backwards, like it did in Sonic & Knuckles. It will also occasionally shoot metal quills in multiple directions.
Sonic the Fighters
In Sonic the Fighters, a new rocket-like Sonic robot appeared briefly in the opening cutscene. From various media it is known as either Rocket Metal or Mecha Sonic Model No.29. This robot is programmed into the game as a fully playable character, though it can only be enabled by hacking. When played as, the robot is called RocketMetal. One of RocketMetal’s moves is to turn into a rocket and shoot forward; this is also one of the standard Metal Sonic’s moves in the game, and accordingly it is generally believed that this robot is a transformation of Metal (rather than a new design of Mecha) Sonic.
When E-102 Gamma is first activated by Eggman, and scans around the robotics lab in Final Egg, two Sonic robots can be seen floating inside green suspension tubes. One is Metal Sonic himself, while the other is a sleek, grey form of the silver Metal Sonic from Death Egg Zone. While never seen active, the robot's design was used for the character of Silver Sonic II in the Archie Sonic comics.
Sonic the Hedgehog (Archie)
This Mecha Sonic attacked Bunnie Rabbot and Knuckles before it started to destroy Knothole Village. It was eventually attacked by Knuckles again, roboticized into Mecha Knuckles. The two battled until it punched Mecha Knuckles clear into Robotropolis toward a stockpile of nuclear warheads. Mecha Knuckles, using the power of magnetization, attracted Mecha Sonic, causing both of them to fall into the stockpile. The explosion of the nuclear warheads greatly damaged Mecha Sonic, but left it active enough to retrieve Sonic's life force to return him to normal.
This Mecha Sonic's body contained rocket-propelled feet, a communication device on its wrist along with laser cannons, optic lasers in its eyes and a "brain burn-thru" device to ensure compliance. Robotnik also developed the body to enhance the speed and strength that Sonic originally had.
The design and workings of Mecha Sonic would later be used by Dr. Robotnik to develop an improved Mecha Sonic he called Silver Sonic.
In Sonic Quest 3 of the Sonic the Hedgehog Archie Comic series, Silver Sonic is a robot designed by Dr. Robotnik. It is an upgraded version of Mecha Sonic, using its design as a basis and building up on its strength and skills to create a better robot. It was used as one of Robotnik's last resort plans on board his Death Egg, in case an emergency came up.
Such an emergency came up when Tails and Sonic infiltrated Robotnik's Death Egg, destroying its master controls and sending it on an uncontrollable course into space. Robotnik, from his auxiliary control room on the station, summoned Silver Sonic to defeat the two Freedom Fighter intruders.
Although Silver Sonic's strength is considerably more than that of Sonic, notably being able to absorb many hits from him, it has a weakness to electricity. Sonic was able to use a broken live wire and attach it to Silver Sonic, incapacitating it. With Silver Sonic indisposed, Robotnik used his "Plan E", which involved him dressing up in a suit connected to the strength of the Death Egg itself.
Sonic was able to hold off Robotnik by entering Silver Sonic and using it as a battle suit to fight Robotnik. Despite the strength of the Silver Sonic suit, both Robotnik and Sonic were evenly matched in their battle. Sonic was eventually able to launch Robotnik many decks away, giving him and Tails a chance to escape the station. They jumped off the station, using the Silver Sonic suit as protection against atmospheric re-entry. Once past the atmosphere, Sonic ditched Silver Sonic and returned to the surface with Tails.
This robot's design is likely based off of the silver Mecha Sonic from Sonic 2, albeit much larger.