Dolphin

Dolphins were a type of Cetacean native to Earth which were known for their intelligence.

Culture
Dolphins were roughly as intelligent as humans, but more playful and with a predatory mindset. (PDA: Island of Death) They were also a very quarrelsome species, and had more than three hundred words for insulting members of their own family. (PDA: Storm Harvest) They liked to swim alongside boats. (DWM: Voyager)

History
While sailing in the Indian Ocean, the Third Doctor spent most of the trip conversing with a school of dolphins. When Sarah Jane Smith was pushed overboard, the Doctor was able to get help from a passing family of orcas. (PDA: Island of Death)

Eventually, humans learned to communicate with dolphins. They were made translator units to talk with and robotic suits to move around on dry land. After the Earth became heavily polluted, many dolphins were moved to the poles, where it was cleaner. Dolphins also moved off the planet to ocean worlds like Coralee. As much as a quarter of the population of these worlds were dolphins. (PDA: Storm Harvest)

The Seventh Doctor and Mel once went to a research centre specialising in human/dolphin communication. There, they encountered a dolphin named Kawili'kai, who was accused of murdering the centre's human personnel. When the Doctor gave him a translator unit, he explained that the "murder" was self-defence. Fearful of humans, he was prepared to die if it meant preventing wide-scale exploitation of dolphins on Earth. Before the conflict came to a deadly conclusion, a dolphin ambassador emerged from the Thames and began the difficult task of inter-species diplomacy. (ST: Driftwood)

References to dolphins
While planning to take a sky shark to pull a carriage, the Eleventh Doctor stated, "Don't think shark, think dolphin". Abigail Pettigrew contradicted him, saying the two were nothing alike. (DW: A Christmas Carol)