Monster X

Monster X is a skeletal monster from Godzilla: Final Wars, serving the Xiliens. His attacks include gravity beams from his eyes, melee attacks, and transforming into the more powerful Keizer Ghidorah. He is also shown briefly to levitate for a short period of time. Even before transforming, he's still extremely powerful. He's a physical match for Godzilla and his gravity eye beams posses roughly the same strength as Godzilla's thermonuclear breath. He's also the only monster Godzilla fought in Godzilla: Final Wars that survived getting blasted by Godzilla's thermonuclear breath at point-blank in the face (actually appearing to shrug the attack off), as well as being the only one able to bring Godzilla to his knees, and make him scream out in pain, with just one attack.

Godzilla: Final Wars
Monster X is a kaiju featured in 2004's Godzilla: Final Wars. He serves as Godzilla's final, most powerful opponent in the film.

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 both were 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 Kaiser Ghidorah.

Trivia

 * It is possible that Monster X's name is a reference to Monster Zero, another alias for the lord of terror, King Ghidorah.
 * Monster X is also used to refer Guilala.