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 so one must defeat the Heartless before riding the Dolphin to reach the Shipwreck.

Kingdom Hearts
In Kingdom Hearts, the Dolphin plays a really minor role, only leading Sora and company to Shipwreck. It can also lead Sora to other places in Atlantica if the player decided to let go of the Dolphin before reaching the Shipwreck. Its help is mostly not needed after Sora and company gets the Mermaid Kick ability.

Kingdom Hearts II
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 missions.