Plesiosaurus

The Plesiosaurus was a large marine reptile which existed in the seas of Earth during the Age of the Dinosaurs. Like the Ichthyosaurus, it was not technically a Dinosaur itself, but, according to the Fourth Doctor, could be considered an "honorary" Dinosaur. (PROSE: The Doctor Who Dinosaur Book)

Biology
Plesiosauri were distinguished by a small head, long and slender neck, broad turtle-like body, short tail, and two pairs of large, elongated paddles. The Plesiosaurus shown in Vorg's miniscope was brownish green and was mildly aggressive. It was intelligent enough to be controlled by the miniscope. (TV: Carnival of Monsters) Plesiosauri fed principally on fish. (PROSE: The Doctor Who Dinosaur Book)

History
The Fourth Doctor caught a glimpse of a pair of Plesiosauri while running from an angry Pteranodon, while around 75 million years prior to the 20th century. (PROSE: The Doctor Who Dinosaur Book)

One Plesiosaurus was transplanted by Vorg into his miniscope, where it menaced the Third Doctor. (TV: Carnival of Monsters)

In an alternate universe, the plesiosaurus was one of the creatures bought back from extinction by the Silurians. They were used as mounts by the Sea Devils. (PROSE: Blood Heat)

Although Plesiosauri were thought to be extinct by the 20th century, the Doctor once wondered if sightings of late-surviving Plesiosauri might have been at the root of some of humanity's legends about sea serpents. (PROSE: The Doctor Who Dinosaur Book)