After having battled Godzilla, King Ghidorah's middle head was blown off by Godzilla's blue atomic fire breath. Ghidorah was later revived in the far future and brought back to 1991 to fight Godzilla again as MechaGhidorah.
In the movie, "Godzilla vs. MechaGodzilla 2", scientists take the robotic head of MechaGhidorah, and use it to create MechaGodzilla 2.
For the full English dub movie of "Godzilla vs. King Ghidorah", please click here - [link]
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MechaGhidorah art video - [link]