Pliosaur

The pliosaur was a creature from the Jurassic age of Earth's history. Pliosaurs were carnivores. The pliosaur was described by Osgood as "a whale with a crocodile's mouth". The massive creature could be found in the area which would later become Dorset.

One pliosaur on the Jurassic coastline caused temporal fallout while pockets of temporal instability were affecting Wychert's coast across time. It came through to the 2010s, and was attacked by UNIT. Osgood thought it was "beautiful", especially due to its remarkable size. The pliosaur ate at least one of their men before spontaneously ageing and getting reduced to bones. On contact, the bones turned to dust. (AUDIO: The Sacrifice of Jo Grant)

Behind the scenes
Though Lieutenant Wallace exclaims that it's a dinosaur before Osgood correctly identifies the creature, in real life pliosaurs are not, strictly speaking, dinosaurs.

In the real world, one of the largest pliosaur specimens ever found was discovered on the Jurassic Coast near Weymouth, Dorset.