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The Fish was an inhabitant of the Razor Reef, and the guardian of the Diamond Cave, an entrance to the World Beneath. It is none other than

Dunkleosteus, a form of giant prehistoric fish and the last Dunkleosteus in Dinotopian seas. Despite this, it more closely resembles an eel in appearance, with wide eyes and sharp curled teeth. It was first seen from a distance, following Frank Scott's plane as it sank beneath the waves for the simple reason that the plane-crash disturbed it.

Later, as Cyrus Crabb, David Scott, and Karl Scott prepare for their expedition to the World Beneath, David questions if Mosasaurus and Pteranodons guard the North Entrance to the World Beneath in the temple at the swamp (where Cyrus possibly lost his leg trying to find a way to the World Beneath which is why he warns the Scotts not to return there after their last time there) then what guards the South Entrance. Cyrus walks past them with a large harpoon, and when asked by Karl: "What the hell is that for?" Cyrus ignores him, singing to himself. David says his point has been proven. It is implied that Cyrus knows about the Fish since he heard Zippo say that something guards the southeren entrance. 

When the submersible, the Reginald Volly passes by the wreckage of the Scott's plane, Cyrus remarks about what may have happened to Frank: "The Fish would've had..." before being told to shut up by Karl. As they pass through the Diamond Cave, the Fish watches them from the nearby Razor Reef.

Later on, when Cyrus betrays the boys, and tries to escape, the Volly's power dies. As he desperatly tries to find a sunstone that fits in the engine, the Fish passes by the Volly's window, its eyes reflected in the Volly's lights. Cyrus, horrifed, says "No...not like this!" The Fish then charges directly at the Reginald Volly. It devours Cyrus and tears straight through the Volly to the other side, the Volly falls apart after this, only rubble remaining. On the surface, the Scotts only see the wreckage float up, including Cyrus' wooden leg.

It makes one appearance in the TV Series nearing the end of the episode, "Making Good" when David on Free-Fall throws the containment case that holds the T. rex controlling pendant and blocks its powers into the sea, in the Razor Reef. The Fish swims up and eats it.