Monster X is a kaiju featured in 2004's Godzilla: Final Wars.
Monster X's name is a reference to Monster Zero, another alias for the lord of terror, King Ghidorah.
Monster X was summoned by the Xilians as their trump card against Godzilla, coming to earth inside a meteor. Godzilla and Monster X commenced battle, with Monster X holding an initial advantage, and then fought to a standstill. Mothra arrived to even the odds, and in response, the Xilians sent out the upgraded Gigan, who quickly dispatched her, and then assisted Monster X. Monster X and Gigan were briefly able to overcome Godzilla, when Mothra attacked once more and knocked both of them down. Gigan resumed his battle with Mothra, and was killed. Although Monster X had the advantage Godzilla soon seemed to gain the upper hand as he knocked the other monster down and punched him multiple times. Finally they split apart and Monster X and Godzilla locked beams, causing a massive explosion which blew the kaiju off their feet.
Godzilla staggered to his feet, and looked up to see Monster X undergo a terrifying transformation: he dropped to all fours, grew wings, enlarged his bodily proportions, changed his white armor into a dark golden armor, and then finally, his neck elongated and his shoulder skulls filled out and became a pair of extra heads. Monster X had transformed into Keizer Ghidorah.
After Godzilla and Monster X battled to a stand still, Monster X undergo a terrifying transformation; he dropped to all fours, grew wings, enlarged his bodily proportions, changed his white armor into a dark golden armor, and then finally, his neck elongated and his shoulder skulls filled out and became a pair of extra heads. Monster X had transformed into Keizer Ghidorah. Godzilla, seemingly undaunted at the titanic beast that appeared before him, fired his atomic ray immediately after Ghidorah rose. Ghidorah returned fire with his gravity beams, forcing Godzilla's atomic ray back at him. Godzilla was thrown to the ground, left at the mercy of Ghidorah and his devastating beams. Ghidorah smashed Godzilla across the destroyed landscape, sending him crashing into crumbling buildings and slamming him to the ground with his beams. Eventually, Ghidorah stomped Godzilla, and kicked him into a building, before seizing Godzilla in his jaws and nearly absorbing all of his life-force.
It is at this point when the mutant soldier Ozaki, realizing Godzilla is their only hope at saving the planet, leaps into the main firing chair of the warship Gotengo and uses his own Keizer powers, an ancient energy procured from the Xiliens, to transfer a massive amount of energy to Godzilla. With this power, Godzilla was able to send a pulse of energy throughout its body, releasing Keizer Ghidorah's tight grip, and breaking of his three head off in the process. The revived reptile fired his atomic ray at Keizer Ghidorah's middle head, causing it to explode. The cunning creature forced the hateful hydra's right head to send a gravity beam scorching into its left, leaving the grotesque dragon mangled and maimed. The limp body of the once tyrannical titan was hurled into the sky, and a hyper spiral atomic ray followed. The motionless body flew far into the atmosphere and erupted into a million pieces.
Monster X's attacks include gravity beams from his eyes, melee attacks, and transforming into the more powerful Keizer Ghidorah. Keizer Ghidorah can fire gravity bolts out of each of his three heads, and can also unleash a high telekenetic blast (much like that of Space Godzilla's) that allowed him to lift and throw Godzilla. His bolts, when fired from all three heads, are more powerful than Godzilla's only radioactive blast. His last ability is to bite onto his enemies and drain out their lifeforce. Because of his great strength and the fact that he drains the very life of his foes, breaking the grip is nearly impossible. Monster X was already an extremely deadly monster as, but as Keizer Ghidorah he's even more powerful.