Harry Bernard's dolphin sculpture
Dolphins were toothed cetaceans native to Earth.
Species of dolphin included the bottlenose dolphin and the pink dolphin. (Star Trek IV: The Voyage Home; ENT: "Fight or Flight")
In 2364, Harry Bernard created a sculpture of a dolphin. (TNG: "When The Bough Breaks")
There were dolphins aboard the USS Enterprise-D. Geordi La Forge showed some of them to Par Lenor in 2368. (TNG: "The Perfect Mate")
The Star Trek: The Next Generation Technical Manual indicates that the dolphins were part of the ship's complement, serving as navigational specialists. This would indicate that they are in fact, a sentient species. They likely operated out of Cetacean Ops.
In 2369, dolphin was a word which appeared on a medical analysis, seen by Doctor Julian Bashir who was infected by the aphasia virus. (DS9: "Babel")