King Ghidorah is an alien three-headed wyvern who is the arch nemesis of Godzilla. He breathes electric Gravity Beams and is a very deadly foe. He's other names are Monster Zero and The One Who is Many.
His first appearance was in the Showa series. Ghidorah was quite a threat and was barely defeated by mothra, Rodan, and Godzilla. The next movie Alien's tricked earth to release Rodan and Godzilla on their planet in zero gravity to battle King Ghidorah only to get mind controlled. The aliens released the monsters upon earth to wreak havoc. The two monsters broke free of their mind control and then fought Ghidorah once more. Many movies later he appeared in Godzilla vs Gigan and teamed up with the chicken-like creature named Gigan against Godzilla and Anguirus.
King Ghidorah was a group of small creatures called do dorats mutated by time travelers to wreak havoc against Japan. Godzilla is there to save the day but was nearly killed until the time travelers flying base knocked ghidorah down. Godzilla ripped two of King Ghidorah's heads off. Later he was revived into a Cyborg to have a final fight with in the city. In a later Heisei movie, King Ghidorah was battling a form of Mothra Leo.
King Ghidorah appeared as a guardian of the earth along with Baragon and Mothra against an evil Godzilla. After the trio of heroes were defeated, their spirits finished the fight, nearly killing Godzilla.
King Ghidorah was an alien who was frozen in antarctica a long time ago. Dr. Emma Russel unfroze him to try to turn human damage around, but Ghidorah awoke all of the titans with an alpha call. The military dropped the Oxygen Destroyer on Godzilla and Ghidorah, but Ghidorah stayed alive because he was an alien who didn't need to breath. In the end, Godzilla killed him with thermonuclear pulses.