Sea serpent

There were legends about sea serpents on Earth among humans; the Fourth Doctor theorised they may have grown from sightings of late-surviving Plesiosaurs. (PROSE: The Doctor Who Dinosaur Book)

However, at some point prior to his eighth incarnation, the Doctor encountered a very real species of intelligent sea serpents living on a different planet than Earth. (PROSE: Vampire Science)

Culture
The sea serpents worshipped beings from their planet (whom the Doctor likened to Earth's whales) as gods. The whales in question were not sapient beings, and the serpents were aware of this by the time the Doctor encountered them; however, they rationalised that the whales were so "enlightened" that they did not need any kind of superior intelligence. (PROSE: Vampire Science)

History
In the 20th century, the sea serpents existed on the fourth planet from a red star that was easily visible in Earth's night sky. To prove that he was truly a proficient interstellar traveller, the Eighth Doctor pointed it out to Carolyn McConnell and told him what he knew of its inhabitants. (PROSE: Vampire Science)