Crassigyrinus

Crassigyrinus (meaning "thick tadpole") is an extinct genus of carnivorous stem tetrapod that lived during the Early Carboniferous period in what is now Scotland.

Portrayal in the series
Crassigyrinus scoticus is a two-meter (6 ft.) long prehistoric stem tetrapod from Carboniferous Scotland. Unlike Proterogyrinus from Walking With Monsters, its legs were short and stubby, meaning it could not leave the water.

Crassigyrinus was perfectly adapted to life underwater as it had gills to "breathe" underwater and a strong, powerful tail, which it used to propel itself through the water. Its favorite food was fish and other amphibious tetrapods, but it may have also preyed on the giant arthropods of its time, such as the giant griffinfly Meganeura, as well. Crassigyrinus had a pair of large eyes, suggesting that it was either nocturnal, or lived in very murky water.

The Bug House
A Crassigyrinus attacked and bit Nigel while he was traversing the Carboniferous swamps. However, it remained unseen due to the murky water. Later, Nigel caught a Crassigyrinus after he successfully captured a Meganeura. However, since he had no way to transport it back to Prehistoric Park, nor was the Bughouse going to have a built-in pond, Nigel released the Crassigyrinus back into the wild after nicknaming it 'The Swamp Monster'.

Errors

 * Crassigyrinus would have already been extinct for around six million years by the Pennsylvanian epoch, when the episode took place.
 * Crassigyrinus wasn't an amphibian, but a primitive tetrapod.