Death Ghidorah was an extraterrestrial dragon responsible for the destruction of the civilizations of Mars and the extinction of the Elias (the tiny humanoids who served Mothra's species) before finally being defeated and sealed into a tomb by the anscestors of Mothra.
Death Ghidorah was an extraterrestrial dragon responsible for the destruction of the civilizations of Mars and the extinction of the dinosaurs (though this is disputed in "Rebirth of Mothra III" where it is said that King Ghidorah was responsible). He was defeated and sealed into a tomb by the anscestors of Mothra.