Mechagodzilla

Mechagodzilla is a robotic monster created by a race of ape-like aliens (also known as Simians) in the image of Godzilla and the main antagonist from Godzilla vs. Mechagodzilla and Terror of Mechagodzilla.

Godzilla vs. Mechagodzilla
Mechagodzilla launched an assault on the Earth in a Godzilla disguise of pseudo flesh covering. Anguirus wasn't fooled by this masquerade and attacked Mechagodzilla. However the kaiju was defeated and forced to flee after having his lower jaw snapped. However the fight did tip off the humans to Mechagodzilla's farce after Anguirus scraped off part of his disguise and exposed the metal underneath. Eventually Godzilla arrived to challenge his doppelganger and blasted off the rest of Mechagodzilla's disguise with his atomic fire breath. The bionic monster then unleashed his full abilities on Godzilla, easily overwhelming his organic counterpart, but was called back for repairs due to damage the King of the Monsters had inflicted on him. Mechagodzilla was sent back into action after its creators found the location of the guardian kaiju, King Caesar. They hoped to destroy him before he could awaken and stop their plans, but Mechagodzilla failed to do so. The bionic monster engaged King Caesar and initially had trouble fighting him, due to the guardian kaiju's ability to reflect the robot's eye beams, but was eventually beaten senseless by Mechagodzilla. Godzilla joined the fight to try and turn the tide, only for the robotic terror to let loose its whole arsenal on its two opponents. However the King of the Monsters generated a magnetic field around himself (it isn't explained how this was possible, but it may have been due to the lightning bolts that struck him earlier in the movie). Mechagodzilla tried to fly away from Godzilla, but the field was too strong and pulled in the bionic monster. The King of the Monsters managed to take hold of the robotic terror and twisted off his head (which is strange considering the fact that it demonstrated the ability to easily spin its head around earlier). Mechagodzilla exploded, scattering its remains into the ocean.

Terror of Mechagodzilla
While Mechagodzilla was defeated in the previous film, the Simians weren't finished yet. The Simians rebuilt their machine titan and, after seeing the power of teamwork, called in an old debt to team Mechagodzilla with Titanosaurus. The aquatic dinosaur first tried to fight Godzilla alone, but was defeated while Mafune, a mad scientist, began rebuilding Mechagodzilla. He believed that the bionic monster would allow him to bring about his revenge on his own species, man, for all the other biologists thinking he was crazy. Mafune believed that he wouldn't have to obey the aliens since he thought that Titanosaurus would fight Mechagodzilla and emerge victorious due to the dinosaur having an organic brain, thus giving it an advantage over the machine. The aliens promised Mafune that they would build a new home for him and his daughter, Katsura, from the ruins of Tokyo with highly advanced technology. However she was rebuilt as a cyborg in control of the two monsters by the aliens and Mechagodzilla and Titanosaurus engaged Godzilla. The King of the Monsters attempted to tear off the robotic terror's head to defeat it again, but the bionic monster was able to continue to fight. However Katsura let herself die so as to stop Mechagodzilla and with her gone, the robotic terror was unable to move. Godzilla destroyed the bionic monster with his atomic breath and Titanosaurus was soon defeated as well.

Abilities
Mechagodzilla possesses enormous strength and is made of "Space Titanium." It is equipped with an enormous arsenal, including eyebeams, an energy beam fired from its chest, and missiles on its fingers, knees, and toes, which can also funciton like darts. Mechagodzilla can also spin its head around to generate a powerful shield and fly at mach 5.

Trivia

 * In Godzilla vs. Mechagodzilla, the initials on Mechagodzilla's arms were originally labelled "MG", which stands for "Mecha Godzilla", while in Terror of Mechagodzilla, after Mechagodzilla was rebuilt, the initials on its arms were labelled "MG2", which stands for "Mecha Godzilla 2", and thus it is given the name "Mechagodzilla 2".
 * The later incarnations of Mechagodzilla that followed, after Terror of Mechagodzilla with Mechagodzilla 2, were human-built and are portrayed in a more heroic light.