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 are also known to be a very quarrelsome species, and have over 300 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 later pushed overboard, the Doctor was able to get help from a passing family of orcas. (PDA: Island of Death)

Eventually, the humans learned to communicate with dolphins. They were made translator units to communicate 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. Many dolphins also moved off the planet to ocean worlds like Coralee, and as much as one-fourth 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)