Monster X was a Space Kaiju created by the Xilians to serve as their secret weapon during The Thousand Year War alongside the Nebulans and Devonians the against the Kilaaks, Simians and the Millennians before being freed following their demise during the war. He also possesses an alternate personality named Kaiser Ghidorah which can take a physical form through body transformation. He joined several other Space Kaiju in attacking Terra, but was killed by Godzilla.
In his normal form, Monster X is a bipedal humanoid/dragon like creature. His body has an exoskeleton-like armor skin which gives him his skeletal appearance with the bony armor being almost white which the rest of his skin is black. On his shoulders he posses two half-skulls. His tail is long and black, with it being forked at the end. On his skeletal head are two long horns and he possess red eyes, even the half-skulls on his shoulder have an eye each.
As Kaiser Ghidorah, his body incorporates traits from the original King Ghidorah as well as DesGhidorah. Kaiser Ghidroah is quadrupedal as Monster X's arms form the front legs. His skin is golden in color though it is a darker shade than that of King Ghidorah as well a having bluish streaks in some areas and replacing scales for a more smoother skin. Some areas of his body have spikes erupting from the sin such as the knees of his front legs and on the necks. Much like the other Ghidorah's, Kaiser Ghidorah has three heads as the half-heads on Monster X's shoulders form the left and right head. The central head has a longer neck than the other two and unlike other Ghidorahs, each head has a different horn arrangement: The central head has two horns above its head and has a fin-like crest in between them moved further back, he right head simple has two horns above the its head with multiple spikes running from the head to the neck and the left head has three horns(the central horn is large than the smaller side horns) and has numerous spikes running down the top part of the neck (Though not as many as the right head). The forked tail of Monster X has now split into two tails and from the back sprouts a pain draconic wings, however in the case of Kaiser Ghidorah, these wings as smaller compared to King Ghidorah and the original wings of Mecha-King Ghidorah.
Monster X is an honorable fighter, but is a fighter first and honorable second. If he encounters an opponent that has the capability to kill or harm him and they are in his way, he will not hesitate to put them down. However, he is not evil by choice, but most often acts when following orders from those who claim to be his masters (much like Gigan) and seems to have qualms about fighting or killing those who can't stand a chance to fight back.
Kaiser Ghidorah on the other hand, is an utter sociopath with almost no qualms or restraints whatsoever. If he gets even the slightest sense that someone or something is in his way, he will not hesitate to kill it regardless if the subject in question is friend or foe. Any of the rare occasions Monster X's personality flares up, its usually comparable to X with his restraints or inhibitions turned off.
Monster X's past is mostly shrouded mystery, much like King Ghidorah. All that is known about him is that he was created by the Xilians to serve as their greatest weapon during the Thousand Year War alongside the Nebulans and Devonians to due battle against the Kilaaks, Simians and Millennians.
At somepoint during the war, Monster X was sent to the Devonian home planet when the Kilaaks invade the ocean planet. Monster X engaes in battle with the Kilaak's kaiju, Hedorah. However, the battle between the space monsters eventually results in the destruction of the Devonians' planet. Near the end of the war after the Xilians were destroyed, Monster X wander aimlessly around the galaxies. In the modern era, he came across SpaceGodzilla, who recruited him to help him conqueror Terra.
Monster Hunter Arc
In 2008, Monster X comes to invade Terra alongside SpaceGodzilla, Gigan and Hedorah, with Monster X taking the form of the large meteor as they travel through space. As they enter Terra's atmosphere the four space monsters split up and attack different cities across the globe. Monster X lands in New York City and takes on his skeletal-kaiju form. As Monster X is about to destroy the Empire State building he is stopped by Mothra. The divine moth uses her Mana Bolts to gain the upper hand but Monster X fires his Gravity Beam at Mothra, causing her great pain and sending her crashing into the Brooklyn Bridge, taking down the divine moth. Monster X then continues to decimate the entire city. Soon Monster X is confronted by both Kiryu and Kumonga. Monster X is pinned by Kumonga's webbing but managed to free himself before he grabs Kumunga. Monster X begins to squeeze the spider's body in hands until his joints pop and he then throws the crippled spider into Kiryu. As the alien and the mecha battle, Monster X finishes Kiryu off by firing his gravity beams at it, sending it into the Hudson River.
Soon Godzilla and SpaceGodzilla arrive on the scene. As Godzilla's Atomic Breath and SpaceGodzilla's Corona Beam clash, Monster X fires his Gravity Beams at where the two attacks meet, causing a great explosion and sending all three flying back. After Godzilla sends SpaceGodzilla into retreat, Monster X faces the King of the Monsters. Monster X begins to gain the upper hand in the fight until Godzilla hits him with his Atomic Breath. However as Godzilla begins to recover, Monster X drops on all fours and his body begins to tremble. Monster X's body begins to transform into the more powerful Kaiser Ghidorah.
With his transformation fully complete, Kaiser Ghidorah fires his Anti-Gravity Bolts at Godzilla and manages to overpower his Atomic Breath, sending the saurian flying across the Hudson River and crashing into to the Statue of Liberty. Before his opponent can get up, Kaiser Ghidorah fires again and sends Godzilla crashing into the Manhattan Bridge. Godzilla soon recovers and blasts Kaiser Ghidorah with the Atomic Breath. Angered, Kaiser Ghidorah fires his Anti-Gravity Bolts at Godzilla and actually lifts him off the ground and starts to lift him higher and higher into the air with said attack. When Godzilla is high enough, Kaiser Ghidorah powers off his attack and lets Godzilla crash to the ground. Kaiser Ghidorah crashes on Godzilla feet first. Before Kaiser Ghidorah can attack again, Rodan arrives to help Godzilla. Rodan attacks Kaiser Ghidorah from behind his head using his talons but Kaiser Ghidorah retaliates. Unfortunately for the space monster, Rodan uses his Sonic Wing Beat to actually decapitate Kaiser Ghidorah's central head. Kaiser Ghidorah uses his remaining two heads to blast Rodan with Anti-Gravity Bolts, sending the pternadon soaring like a comet. However Godzilla recovers and attacks Kaiser Ghidorah. He uses his left head to bite Godilla's throat as the right launches towards Godzilla. However Godilla grabs the right head and forces it to bite the left head off. Kaiser Ghidorah is then forced down by Godzilla's Atomic Breath and Godzilla stomps his foot on the last remaining head and crushes its skull, killing Kaiser Ghidorah.
Post-Trilopod War Arc
While Monster X himself doesn't appear, he appears in Godzilla's memories as the Shobijin show the core members of Fairy Tail Fairy Tail after the Earth Defenders were transported to Earth Land. As the Fairy Tail mages witness Godzilla's memories, they see Monster X in its battles against Godzilla, Mothra, Rodan, Kumonga and Kiryu. They also witness the transformation of Kaiser Ghidorah.
Gravity Beams: Monster X is capable of firing thunderbolt-like energy beams from his eyes: both the two eye on his central head and one each from his half-heads on his shoulders. This is incredibly power and is able to blast away Mothra and Kiryu.
Ant-Gravity Bolts: As Kaiser Ghidorah, he is capable of firing thunderbolt-like energy beams from his three heads. These energy beams have an additional effect of telekinesis, if they strike an opponent, Kaiser Ghidorah is capable of lifting of the ground and high into the air. By powering down his attack, Kaiser Ghidorah can release his victim from this telekinesis.
Immense Strength: Monster has shown to posses great physical strength as he can match Godzilla in physical combat. His is strong to cripple Kumonga by seeing his body until his joints popped. As Kaiser Ghidorah, his physical strength increases greatly.
Immense Durability: Thanks to his bony armor, Monster X is able to survive powerful energy attacks from his opponents include those from Godzilla, Mothra and Kiryu.
Flight: As Monster X, he can travel through the vacuum of space in the form of a meteor. As Kaiser Ghidorah he can use his wings for flight.
Transformation: Monster X's alternate personality known as Kaiser Ghidorah can manifest through the physical transformation of Monster X's body.