Sea serpent

A species of intelligent sea serpents were once encountered by the Time Lord known as the Doctor, at some point prior to his eighth incarnation.

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)

Behind the scenes
The Doctor describes the inhabitants of the planet as "sea serpents" with the apparent presumption that this would give Carolyn McConnell a broad idea of what they look like; this is a reference to the existence of (usually non-sentient) "sea serpents" in real-life human folklore. However, there are, to Tardis's knowledge, no other reports of myths of sea serpents in the Doctor Who universe.