Bottlenose dolphin
From Memory Alpha, the free Star Trek reference
The bottlenose dolphin (Tursiops truncatus) is an aquatic mammal indigenous to Earth. It is a cetacean species.
When Spock tried to match the sound of the whale probe to the sound of an Earth animal aboard the Klingon Bird-of-Prey HMS Bounty, a bottlenose dolphin was one of the animals shown. (Star Trek IV: The Voyage Home)
Harry Bernard successfully carved a wooden statue of a bottlenose dolphin with a device that emitted a pink/orange beam while in the forced custody of the Aldean natives Accolan and Leda. (TNG: "When The Bough Breaks")
The Star Trek: The Next Generation Technical Manual indicates that the dolphins were part of the Enterprise-D's complement, serving as navigational specialists. This would indicate that they are in fact, a sentient species. They likely operated out of Cetacean Ops.
