The Giant Turtle is a character from Majora's Mask. Although the Giant Turtle does not appear for a particularly long amount of time in this game, riding on the turtle's back is necessary to progress through the game, specifically, getting to the Great Bay Temple.
Entering the Great Bay Temple
Once Link (wearing the Zora Mask) and Lulu are outside Zora Hall at the Zora Cape, he must play the "New Wave Bossa Nova" to awaken the submerged Giant Turtle. The gargantuan beast rises from the water and sports two conveniently-located trees that Link can Hookshot to. This causes the Giant Turtle to begin swimming to the Great Bay Temple.