From the Super Mario Wiki
Mechakoopas are one of Bowser's many minions. They are small mechanical versions of Bowser himself, resembling a wind-up toy. They have a wind-up key on their back and their appearances feature several mechanical appendages. They walk in a rhythmical way and must wind themselves up again from time to time in order to keep moving.
Super Mario series
Super Mario World
Mechakoopas make a short appearance in Super Mario World, where they are only seen in the last level. In this game, they get paralyzed for a short time when stepped on, so Mario can kick them around, pick them up and throw them. When they get thrown, however, they won't get very far like regular Koopa shells do. They are also not affected by ordinary kicks, as they just freeze, giving the player a chance to throw them somewhere; however, they are vulnerable against the Spin Jump, which turns them into dust.
Super Mario Galaxy
Mechakoopas also appear in Super Mario Galaxy with a new look. They are found in Toy Time Galaxy and can breathe flames for fairly long amounts of time. They can even chase Mario which Goombas do as well. They can only be defeated with a Ground Pound and stunned with a Star Spin. When defeated, they release a coin. In this appearance, they are called Mecha-Bowsers.
New Super Mario Bros. Wii
Mechakoopas (also known as Windup Koopas or Bowser Bots) return in New Super Mario Bros. Wii, where they were found on Airship levels. In this game, they have their appearance from the Mario Party games, and have the same attack pattern as they had in Super Mario World. Unlike in Super Mario Galaxy, they can't breathe fire.
It is shown, that through hacking methods, that Yoshi can eat a Mechakoopa. However, he can't jump on one.
Super Mario Galaxy 2
Mechakoopas return in Super Mario Galaxy 2, sporting their looks from Super Mario Galaxy. Here, they appear in the Flipsville Galaxy during the mission "Flipsville's New Digs" in the first section of the galaxy, and in the Cosmic Cove Galaxy during the mission "Exploring the Cosmic Cavern" on the last planet. In addition to the Ground Pound, Mario can also use the Spin Drill to defeat the Mechakoopas by jumping over them and then drilling into them.
New Super Mario Bros. U
Super Mario World cartoon
Mechakoopas, referred to as Mechkoopas, and sometimes generically called "robots", appeared in several episodes throughout the Super Mario World animated series.
Their first appearance is in the episode "Send in the Clown", where a Mechkoopa riding a bicycle acts as the tight-rope walker in Bowser's circus, the "Koopaling Bros. Circus". After the circus is revealed to be a trap, this Mechkoopa, along with several Chargin' Chucks, chase Mario through Neon Castle. The Mechkoopa is eventually defeated by Mario, who gains the cape ability. Later in the episode, Bowser attempts to defeat Mario by throwing several Mechkoopas at him from the Koopa Clown Car. Mario, after knocking a Mechkoopa unconscious, defeats Bowser by hurling the Mechkoopa at the car, causing it to malfunction and crash in a very similar way as Super Mario World.
Mechkoopas next make a small appearance in the "The Night Before Cave Christmas", where several of them attempt to attack Mario and Yoshi, who had broken into Neon Castle. Mario and Yoshi escape these Mechkoopas when Mario grabs a Feather to transform into Cape Mario and flies away.
The last appearance of Mechkoopas in the Super Mario World TV series is in the show's final episode, "Mama Luigi". During Luigi's story, Bowser sends a brigade of Mechkoopas after Mario and Luigi as the two explore Bowser's throne room looking for Yoshi. After the Mechkoopas chase them for a while, they are eaten by Yoshi.
Nintendo Adventure Books
In Flown the Koopa, if Mario and Luigi run into Bowser in the Valley of the Koopas, he will sic four Mechakoopas he had hidden in the trash can he was using as a chair on the two. In one possible scenario, Luigi will smash one of the Mechakoopas with his wrench, and make the others destroy themselves (by pressing the sports channel button) after taking their remote from Bowser. In the second scenario, the Mechakoopas will knock the Mario Bros. out with their telescoping robotic arms, then turn on Bowser, knocking him out as well. The Mechakoopas then wander off, intent on enacting their own world domination schemes.
During a fight with Mario and Yoshi, Bowser uses Mechakoopas as projectiles in volume 4 of Super Mario-Kun, similar to his attack in the original game. Yoshi stuffs them in his mouth and spits them back, hitting Bowser.
Super Mario Adventures
In the Super Mario Adventures comic, the Koopalings, after having captured Mario by chance, used two Mechakoopas to send a message to Luigi, Toad and Yoshi. Either they exchange Princess Toadstool for Mario, or Mario will be killed, after saying this, the Mechakoopas symbolically burnt a Mario Doll with the flamethrowers in their mouths. Luigi then said, "That's Blackmail!". Luigi, with help from Friendly Floyd, managed to trick the two Mechakoopas by making them take him to the Koopalings instead of Princess Toadstool.
In Hotel Mario, Mechakoopas (parsed "Mecha Koopas" in the manual) reside in Bowser's Seizures Palace Hotel. They are blue in colour, and trundle around in an odd manner. When stomped, they somewhat shrivel up, and temporarily cease movement. When roughly 3/4 of the stage's time runs out, they simultaneously break down and become immobile piles of junk. This can be an obstacle if one is in front of a door, requiring at least six jumps to be pushed aside. They can be defeated with a fireball.
Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Bowser uses a giant, green Mechakoopa (parsed "Mecha Koopa" in the manual) as a weapon in his attack, Bowser Crush. This Mechakoopa was developed in a top-secret laboratory deep in Bowser's Keep for several months.
Mario Kart series
Mario Party series
Mario Party 5
In Mario Party 5, the player will fight a group of Mechakoopas in the first part of the final battle. They would walk around, occasionally flashing red, then unleashing fiery breath. Jumping on them once will cause them to explode. The player must throw them at each other, or just avoid them all for 60 seconds. Jumping on a Mechakoopa while it's breathing fire will hurt the player.
Mario Party 7
In Mario Party 7, Mechakoopas appear in the one-player, Bowser mini-game, Tunnel of Lava!. Here, one Mechakoopa will eat the key to freedom from Bowser's mini-game area and will retreat into a hole, only to come back a second later with many other Mechakoopas. The Mechakoopas would then circle the character, who would have to stomp on the correct Mechakoopa's head while they circled them before Bowser stomped on the character 30 seconds later. The Mechakoopas had an attack pattern similar to that found in Mario Party 5, except all the Mechakoopas will breathe fire simultaneously.
Mario Party Advance
In Mario Party Advance, a Mechakoopa owned the Mushroom Library, and appeared to be an arithmetic genius. The player must complete the quest "Mathemagician!" for the Mechakoopa in order to get a Gaddget. The quest consisted of answering three simple math problems; after answering the three questions correctly, the player will be awarded with a Gaddget. The ending says that Mechakoopa spends his time creating a new theorem.
Mario Party 9
In Mario Party 9, Mechakoopas are given a minor appearance. They only appear in the minigame Mecha Choice, in which they chase the player through the hallway. They also appear to surround the player if he or she chooses the incorrect door. During the first Captain Event on Bowser Station, Heart Star Colony, if there are less than 4 players, Mechakoopas will fill the open spaces, ensuring that there will always be three players for the captain to match.
Mario Party 10
In Mario Party 10, a giant Mechakoopa is one of the game's mid-bosses. In its boss minigame, players must ride up elevators and swing on suspensions hanging from the ceiling to jump on the Mechakoopa's head and deal damage. As its health dwindles, the Mechakoopa will begin moving more erratically and breathing fire to try and catch players off-guard.
Mario & Luigi series
Mario & Luigi: Superstar Saga
Smaller versions of Mechakoopas make a short appearance in Mario & Luigi: Superstar Saga as conductors to complete circuits of electricity in Bowser's Castle (Unnamed toy mice held the same purpose previously in Woohoo Hooniversity).
Mario & Luigi: Dream Team
Mechakoopas make their second appearance in the Mario & Luigi series in Mario & Luigi: Dream Team. They appear in Dreamy Neo Bowser Castle as common enemies rather than obstacles and retain the same appearance as in other recent games. In the field, Mechakoopas run back and forth rapidly, making them difficult to avoid. In battle they appear in fairly large groups, working together to attack the Bros in two ways. Their first attack method is lining up single-file and either jumping or spitting fireballs, which Mario must jump to avoid. For the second attack they surround Mario from all four directions and charge at him in the order they hop, and Mario hits them away with the hammer. If a Mechakoopa is damaged before or during battle, it will malfunction and reverse its attack pattern. Mechakoopas can give a Bro. the trip status.
Mario Power Tennis
Super Smash Bros. for Nintendo 3DS / Wii U
Mechakoopas serve as Bowser Jr.'s down special move in Super Smash Bros. for Nintendo 3DS / Wii U. They get released from the Junior Clown Car and walk across the arena. If it runs into an opponent it knocks them up into the air before exploding. It will also explode if attacked or simply left on its own, as well as also being able to be picked up and thrown.
Bowser Jr. has two other alternate custom down special moves he can use; Impatient Mechakoopa and Big Mechakoopa. Impatient Mechakoopa launches a Mechakoopa rather than dropping one and it explodes on impact with the ground, dealing less damage than the normal Mechakoopa. Big Mechakoopa is a larger and more powerful version that moves more slowly and walks past enemies rather than exploding on impact.
Names in other languages