The Cryogs are a race of shape-shifting aliens that intended to conqueror Terra in order to find a new planet to live on after their original homeworld was destroyed by King Ghidorah. Their first attempt of their invasion was lead by Commander Rhizon but after his failure the invasion of Terra was taken over by Emperor Karkoro until their destruction during the war between Godzilla and the Earth Kaiju against the Trilopods.
In appearance, the Cryogs are a tall race that have purple coloured skin. Their limbs are tentacles, they have to for their arms and have several for their legs. The tops of their heads are pointed and have red eyes with yellow pupils. Some members of the species appear to have appendages or growths that give symbol of their statues among their society. For example with Commander Rhizon, he has a large fin like mane around his head and chest. Another example is Emperor Karkaro, who also has a fin like mane through his is larger then Rhizon's, Karkoro also has spikes on his mane, his chest and on his head.
Thousands of years ago, the Cryog homeworld was attacked by King Ghidorah. In the attack, the Cryog Emperor Karkoro escapes the planet and abandoning the rest of his people. The survivors where lead by Commander Rhizon as they escaped the planet before it was destroyed by King Ghidorah. With their emperor missing, Rhizon attempts to find another planet to colonize so that his people may have a new home.
At some point, one ship looses it way and gets separated from Rhizon's fleet but it soon comes across Terra and the Cryogs on board see it as a possible new home. However, they soon spot the Millennian UFO, with the Millenians on the space craft also intending on inhabiting Terra. Not wanting competition with another race, the Cryogs fire at the UFO and soon the two alien ships engage in a fire fight. Whilst the Cryogs manage to shoot down the Millennian UFO but their ship is heavily damaged by the Millennians and their engines are shut down, with both ships crashing down towards Terra. Before they crash land, the Cryogs send a signal with coordinates in the hopes it reaches any fellow Cryog ships.
During the battle of King Caesar and Megalon's battle in Okinawa, the Cryog ship crashed into the ocean which created a giant tidal wave that engulfed King Caesar and destroyed the rest of the Ryukyu Kingdom. At some point, the Devonians finds the crashed ship in the ocean and the crew who are still in stasis pods.
Eventually, Rhizon's fleet soon receives the signal from the lost Cryog ship, receiving the coordinates to Terra and the fleet makes their way to find Terra, intending to inhabit the planet.
In 2010, Commander Rhizon and his Cryog fleet arrive at Terra with the intent of colonizing the planet however they find out the humans and kaiju populate Terra and so they in tend to conquer Terra. At some point they made contact with another race of aliens, the sea-dwelling Devonians as they form an alliance to conqueror the planet.
The Cryog observe a blinded and battle damaged Gigan in Arinoza two years after his battle with Rodan and Anguirus. After Gigan's battle with Kumonga, the Cryog managed to take control of Gigan as summon him to their ship to be repaired and ungraded. While the CKR forces are distracted by Juvenile Destoroyah attacking San Diego a team of Cryogs invade a CKR base to steal a sample of Godzilla's flesh which they indent to give to the Devonans. Afterwards, another team of Cryogs along with the upgraded Gigan attack Area 51 where they recover a Millienian UFO. As they are pin pointing Godzilla's location, they observes Godzilla and Mothra battle Destroyah.
They soon tract Godzilla to Las Vegas where they and they send Gigan and the Millennian UFO to face him. They order Gigan to hold down Godzilla while the UFO absorbs Godzilla's DNA until M.O.G.U.E.R.A. arrives to aid Godzilla. Soon when the Millienian UFO absorbs Godzilla's DNA and becomes Orga but the Cryog can't control the new monster as he attempts to eat Gigan. After Jet Jaguar arrives and destroys Orga from the inside out, the Cryogs sends a recovery vessel to recover the still living remains of Orga, Rhizon also states Gigan may not want to retreat yet. As the recovery vessel recovers Orga it accidently abduct CKR members Steven Woods and Chavez. After Godzilla defeats Gigan, the Cryogs call Gigan back to be repaired. Meanwhile, Commander Rhizon receives the coordinates to the Devonians' location from the Devonian King but the crew discover the presence of Steven Woods and Chavez and Rhizon and his soldiers go to confront them. As Rhizon attempts to kill Steven Woods, he is interrupted when Jet Jaguar attacks the ship. Steven Woods manages to freeze one of the soldiers solid and Rhizon retreats, sending in a strike team but the soldiers are overpowered when Jet Jaguar shrinks and helps the humans before the robot carries away the two humans.
The damage ship arrives at the island chain which would become Monster Islands and Rhizon makes contact with the Devonian King, but the king ends the alliance between the two races as he betrays Rhizon by unleashing Biollante onto the Cryog ship. As the ship is attack the Cryogs launch Orga's pod from the ship and things get worse when Godzilla arrives and fires his Atomic Breath at the ship, destroying it before he battles Biollante.
While Lucy Caprell is on Infant Island and is shown the history of the kaiju by the Shobijin, a flash-back shows a Cryog ship crashing into the sea near Okinawa as King Caesar and Megalon are battling and it causes the giant tidal wave that engulfs King Caesar. Later in the flashback, the Cryog ship is sunken and is found by the Devonians who are investigating the wreck and the stasis pods holding the crew.
Rhizon managed to survive the attack and manages to escape into the sea, he reaches San Diego and disguises himself as a Devonians where he encounters a human couple fishing and he asks to meet the human leaders. He is brought to the CKR where he meets Dr Anders, Rhizon fools them into thinking he is a Devonian rebel who is resisting their king and he gives information to the humans in order to destroy the Devonian capital. After the Devonians are destroyed and Godzilla and Jet Jaguar are lost in the destruction, Rhizon reveals his true form to Dr Andres and kills him before he leaves the CKR facility.
Cryogs' Revenge Arc
Four years later, Rhizon disguises himself as a wealthy Russian warmonger and he manages to re-gather his Cryog forces. At some point he discovers the body of Guardian King Ghidorah and also meets the psychic twins Mallory and Minnette whom he acquires their assistances. He orders Gigan to decapitate the middle head of Guardian King Ghidorah and revives the dragon as a cyborg: Mecha-King Ghidorah. He appears in Moscow for a conference as he advertises his new weapons for the humans to use against themselves in the hopes of destroying each other: MechaGodzilla. He states to the humans MechaGodzilla drones are meant to be a cheap alternative to Kiryu, M.O.G.U.E.R.A. and Jet Jaguar as he shows gives a demonstrate a test with Anguirus as MechaGodzilla's target. He manages to persuade a US Senator into using several MechaGodzilla drones against the newly returned Godzilla in Boston. Rhizon has the Senator knocked out as he has the sends two MechaGodzilla drones to take down Kiryu carrying an injured Anguirus and sends Mecha-King Ghidorah to bring Godzilla to Siberia. Rhizon also has Mallory and Minnete put in a machine so they can interface with the cyborg dragon. After Godzilla and Anguirus defeat Mecha-King Ghidorah and MechaGodzilla's, Commander Rhizon and the other Cryogs retreat from their base of operation as the earth kaiju attack, abandoning Mallory and Minette.
Trilopod War Arc
After their defeat by Godzilla and Anguirus, the Cryogs manage to salvage two surviving MechaGodzilla drones but they lost Mecha-King Ghidorah with Rhizon theorising that he called upon another of his kind. The crew detect an incoming object heading towards their ship which is revealed to be SpaceGodzilla. The Cryogs release their two MechaGodzilla units but both are destroyed by the crystalline saurian and the Cryog try to fend off the monster with the ship's weapons. When the weapons are disabled, one Cryog suggests to send a signal for help but Rhizon refuse as he orders Gigan to face the enemy. As the two kaiju battle, the Cryog ship begins to lose powers and the same Cryog ties to send a signal but Rhizon attempts to stop him but in his rage, a signal is accidently sent. After SpaceGodzilla defeats Gigan, the ship witnesses a pod launched past their ship and towards Terra and Rhizon and his crew see that the Emperor Karkaro has returned with a massive fleet and was as the giant hive of the Trilopods.
Karkoro boards Rhizon's ship with the crew happy to see their emperor but Rhizon is still wary of their leader. Emperor Karkoro takes over the invasion of Terra after he takes Rhizon and his crew prisoner and launches more Trilopod hunting pods onto the planet to attack Earth's Kaiju and intends to use them to create Trilopod and Kaiju hybrids. While Rhizon states he was going to use the Earth Kaiju to aid in conquering Terra but Karkoro believes he will use the Earth Kaiju to overthrow him. Karkoro's ship comes closer to the Terra and detaches the Trilopod hive which lands in Godzilla's current location: Las Vegas. Rhizon warns Karkoro not to underestimate Godzilla and Karkoro threatens to sentence Rhizon to death. As Karkoro shows Rhizon Godzilla's trapped by the Trilopods, Rhizon ask Karkoro if he wondered if he thought there would be a more powerful monster than the Trilopods as he states how Karkoro fled from King Ghidorah in the destruction of their home planet and calls him a coward. Karkoro has Rhizon taken to a prison cell and has Rhizon's ship destroyed and crew killed.
When Karkoro tells Rhizon his crew is dead, Rhizon tells that in doing so, Karkoro has let loose an out of controlled Gigan. Karkoro takes Rhizon to show him Godzilla against an army of Trilopod Hybrids, however the crew scenes something moving within the Trilopod hive (which is revealed to be King Caesar freeing the captured Earth Kaiju). Outside of Terra's atmosphere, Gigan destroys Karkoro's fleet and in the confusion, Rhizon manages to take Karkoro hostage. As the Earth Kaiju battle the Trilopods, Karkoro manages to unleash their queen Magita against the kaiju. With Jet Jaguar and Gigan battle on top of mother ship, Rhizon manages to throw Karkoro into Gigan's chainsaw, killing the emperor and once the ship crashes into Magita, a massive explosion is created, killing the last of the Cryogs.
Post-Trilopod War Arc
While the Cryogs doen't appear again, their invasion lead by Karkaro is witnessed by members of Fairy Tail as shown in Godzilla's memories by the Shobijin after both the Earth Defenders and the Earth Conquerors are transported to Earth Land.
Shape-shifting: The Cryogs are gifted with the ability to shape-shift into other races as they are able to disguise themselves as other species such as humans and Devonians.
Advanced Technology: The Cryogs posses advanced technology. With this technology they were capable of space travel and controlling the minds of kaiju.