Mechakoopa
Mecha-Koopas are one of Bowser's many minions. They are mechanical versions of Koopa Troopas. They get paralyzed for a short time when stepped on, so Mario can kick them around and pick them up and throw them. When they get thrown however, they don't get very far like regular Koopa shells do. Also, they are technically invincible unless they are kicked or thrown at an enemy because they always reanimate after a few short moments, much like Dry Bones.
They make a short appearance in Super Mario World, where they only appear in the last World. The final boss of the game, Bowser, throws them off of his Koopa Clown Car, the hovering clown machine he occasionally can be seen riding in. Mario and Luigi must throw them up so they hit Bowser's head. They are like a cross between a Koopa and a wind-up toy. They have a wind-up key on their back and their appearances feature several mechanical appendage. They walk in a rhythmical way and they must wind themselves up again from time to time.
Bowser later uses a giant, green one as a weapon in his Super Mario RPG: Legend of the Seven Stars attack Bowser Crush.
In Mario Party 5, the player fights a group of them in the first part of the final battle. They walk around, occasionally flashing red, then unleashing fiery breath. Jumping on them once will cause them to explode. Jumping on a flashing Mecha Koopa, however will hurt the player.
In Mario Party 7 Mecha Koopas appear in the 1-player Bowser mini-game, Tunnel of Lava!, where one Mecha Koopa eats the key to freedom from Bowser's mini-game area and retreats into a hole, only to come back a second later with many other Mecha Koopas. The Mecha Koopas would then circle the character, who would have to stomp on the correct Mecha Koopa's head while they circle him/her before Bowser stomps on the character 30 seconds later. The Mecha Koopas have an attack pattern similar to that found in Mario Party 5, except all the Mecha Koopas will breathe fire simultaneously.
In Mario Power Tennis, Mecha-Koopas make a small appearance in the background of the Bowser's Castle court. A Mecha-Koopa, aptly named Mecha-Koopa, appears in Mario Party Advance. This Mecha-Koopa, apparently an arithmetic genius, will ask the player several simple math questions. If the player answers these questions correctly, the Mecha-Koopa will give them a Gaddget.
Mecha-Koopas 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 that game's Woohoo Hooniversity world.) Organic versions of Mecha-Koopas appear in Mario Kart Super Circuit.
Mecha-Koopas, redisned slightly, are currently set to make an appearance in the upcoming Super Mario Galaxy. So far they have only been seen in Toy Time Galaxy.
Appearances in Other Media
Cartoons
Mecha-Koopas, though renamed Mechkoopas (and occasionally called by the generic name "Robot"), appeared in several episodes throughout the Super Mario World animated series.
The first appearance of a Mechkoopa was in the episode "Send in the Clown", where a Mechkoopa riding a bicycle acted as the tight-rope walker in Bowser's circus, the "Koopaling Bros. Circus". After the Koopaling Bros. Circus was revealed to be an elaborate trap, this Mechkoopa, along with several Koopa Football Players, begin to chase Mario through Neon Castle. This Mechkoopa is eventually defeated by Mario, who had gained the cape ability.
Later in Send in the Clown, Bowser attempts to defeat Mario by throwing several Mechkoopas at him from the Koopa Clown Car. Mario, after knocking a Mechkoopa unconscious, manages to defeat Bowser by hurling the Mechkoopa at his Koopa Clown Car, causing it to malfunction and crash.
Mechkoopas next made a small appearance in the "The Night Before Cave Christmas", where several of them attempt to attack Mario and Yoshi, who have broken into Neon Castle. Mario and Yoshi manage to escape these Mechkoopas by using a Feather to transform into Cape Mario and fly-away.
The last appearance of Mechkoopas in Super Mario World was in the shows final episode, "Mama Luigi". Here, Bowser sends a brigade of Mechkoopas after Mario and Luigi as the two explore Bowser's throne room looking for Yoshi. These Mechkoopas, after chasing Mario and Luigi, are defeated when Yoshi simply devours them.
Comics
In the Super Mario Adventures comic, the Koopalings, after having captured Mario by chance, use two Mecha-Koopas 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 Mecha-Koopas symbolically burn a Mario Doll with the flamethrowers in their mouths.
Luigi, with help from Friendly Floyd, manages to trick these two Mecha-Koopas by making them take him to the Koopalings in place of Princess Toadstool.
