Cryptocleidus were aquatic long-necked reptiles, or plesiosaurs, that went extinct at the end of the Cretaceous period, in 65,000,000 B.C. However, it seems a number of the Cryptocleidus somehow survived extinction.
A Cryptocleidus appeared and attacked H.E.A.T.'s submarine until Zilla Junior appeared and started to fight it, just as H.E.A.T. discovers a Tachyon ship in the bottom of the ocean.
When H.E.A.T. get out of the Tachyon ship, another Cryptocleidus appears. Zilla Junior tries to evade them and eventually hits a rock formation, causing the two Cryptocleidus to be crushed by the rock slide.
During the event of Monster Wars, one of the Cryptocleidus survived and was spotted swiming towards Site Omega. It attacked H.E.A.T.'s boat and was tricked into fighting the Giant Bat. After the fight was broken up, both mutations traveled to Site Omega. Once the aliens began their assault, Cryptocleidus attacked Hong Kong. When the alien control was severed, Cryptocleidus was killed by the Tachyon ship's beams.