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 in the Undersea Valley will be available even if there are Heartless but the player will be unable to ride the Dolphin until the Heartless are defeated.

Kingdom Hearts
During Sora, Donald, and Goofy's first visit to Atlantica, the Dolphin leads them to the Shipwreck. It also leads Sora to other places in Atlantica if he lets go of the Dolphin before reaching Shipwreck. The Dolphin's help is not needed after Sora and company get the Mermaid Kick ability, but it remains swimming around Atlantica.

Kingdom Hearts II
In Sora, Donald, and Goofy's second visit to Atlantica, several Dolphins are present as minor characters, notably during the singing mission- "Under the Sea."