Dolphin

The Dolphin is found in Atlantica in Kingdom Hearts and Kingdom Hearts Final Mix. Flounder calls them "giant fish." It gives Sora and his party a ride past the currents that block the passage to the Shipwreck. The Dolphin will not be available if there any Heartless currently in the Undersea Gorge, so one must defeat them all before being able to advance the story, and reach the Shipwreck.

Other Appearance
In Kingdom Hearts II and Kingdom Hearts II Final Mix+, Dolphins are present in the game as a minor character, notably during one of the singing mini-games.