King Godzilla is a hybrid beast - created and controlled by a mad Japanese scientist called Doctor Oniyama, who intends to take over Earth.
King Godzilla is a fusion of the DNA of four monsters; Godzilla, Battra , Biollante and King Ghidorah. He has the body, tail and head of Godzilla, but with his arms replaced by two of King Ghidorah's heads and with the wings and brain of Battra. King Godzilla is fatter than Godzilla and has a more pronounced breastbone. When his chest is destroyed, it regrows with the head of Biollante taking its place - the head firing Biollante's acidic sap.
He is the last and most powerful enemy monster that Godzilla faces in the manga Godzilla, King of the Monsters.
Following his failure to destroy Godzilla using his previous legion of mind controlled monsters (both originals and hybrids), Doctor Oniyama set to work on his final creation. Combining the DNA of Biollante, Battra and King Ghidorah with DNA he had managed to harvest from Godzilla, he used a growth medium to grow his new monstrosity.
Upon it's birth, Dr. Oniyama named the three headed beast 'King Godzilla' and - with the monster under his control - he mounted the creature and rode upon it as he unleashed the monster's fury against Japan. Godzilla - despite been badly weakened from fighting his numerous battles with Oniyama's legion of mind controlled daikaiju - answered the call to battle as the two titans battled on the edge of a volcano. Godzilla's weakness was evident, however, as he was badly beaten by his superior clone. King Godzilla's level of regeneration allowed him to regrow King Ghidorah's heads every time they were destroyed, he could use both his atomic breath and Ghidorah's gravity beams at once and his flight and strength - which included been able to lift anything as heavy or heavier than him - gave him a great advantage over the Monster King.
G-Force, who were observing the battle between Godzilla and King Godzilla, quickly realized Godzilla was going to need help as he would not be able to defeat King Godzilla by himself - especially after King Godzilla succeeded in ripping off one of Godzilla's hands. Coming together with a plan, they prepared Machine-G (a sleek version of MechaGodzilla that Godzilla had fought earlier and been forced to retreat from) and gave it a new attack - been able to fire an acidic spray, with the acid itself been the corrosive sap that had been collected after Godzilla's battle with Neo Biollante (another of Oniyama's creations).
With the plans in place, the G-Team mobilized and piloted Machine-G towards the volcano. They used Machine-G's maser cannons to blast open King Godzilla's chest - only to be shocked as the head of Biollante grew in its place. King Godzilla then proceeded to attack Machine-G with a powerful aerial slam and then tried to destroy Machine-G by crushing it underfoot, but was distracted by Godzilla resuming his attack - allowing Machine-G to escape.
Once they'd escaped from been destroyed, Machine-G got within range and fired Neo Biollante's acidic sap at King Godzilla. It proved effective, as the sap blinded King Godzilla. Godzilla - in a suicidal attempt to kill his clone - then tackled them both into the volcano. Both monsters were killed in the volcanic magma, with Dr. Oniyama also been killed as a result.
Godzilla, King of the Monsters - Manga