is a fictional character from various films in the Godzilla series, introduced in Godzilla vs. Mechagodzilla (1974). It is Godzilla's mechanical doppelgänger and one of the most popular Toho kaiju. Mechagodzilla is also recognized as one of Godzilla's most powerful rivals (all iterations have at one point or another come very close to killing the King of the Monsters).
The original Mechagodzilla was created as a weapon of destruction by the Simians.
It first appeared in a pseudo-flesh outer covering, masquerading as the real Godzilla during attacks against Japan in Godzilla vs. Mechagodzilla. Curiously, while the Simians gave Mechagodzilla a laser beam in its mouth to mimic Godzilla's atomic breath, they didn't bother replicating Godzilla's unique roar. Godzilla's ally Anguirus wasn't fooled by the impostor, but in the resulting fight Mechagodzilla broke Anguirus' jaw and sent him fleeing underground. Although the battle went badly for Anguirus, it tipped humanity off to the charade due to the fact that while Godzilla and Anguirus had initially been enemies in 1955 in the second Godzilla film, they had been firm allies ever since, and the two were known to come to one another's aid in combat against other monsters. Anguirus attacking 'Godzilla' was seen as a complete shock. Anguirus had also exposed a piece of MechaGodzilla's true mechanical nature by ripping off a piece of the disguise the machine was covered in, though most humans didn't seem to notice it.
Soon the true Godzilla appeared and exposed his foe's metallic form completely. Interestingly, after losing its disguise, Mechagodzilla's fingers lost all mobility and it lost the beam in its mouth (No explanation for this was ever given). The battle resulted in a tie, however, and in the end it took the combined might of Godzilla and King Caesar to remove Mechagodzilla's head from his shoulders, ending the threat.
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Official Site: Toho Co
Links: WIkipedia entry Monstrous page Toho page