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 hadmore than three hundred words for insulting members of their own family. (PDA: Storm Harvest)

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, and as much as a quarter of the population of these worlds were dolphins. (PDA: Storm Harvest)

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)