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")
During the mid-24th century, Starfleet oceanographer Lieutenant Harry Bernard, Sr. was stationed on Zadar IV with his son Harry. They lived on the beach and Harry Jr. often saw dolphins playing in the ocean. They would later be the inspiration for his first attempt at sculpting. (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")
In 2369, "dolphin" was a word which appeared to be displayed on a medical analysis, seen by Doctor Julian Bashir while suffering the effects of the aphasia virus. (DS9: "Babel")