The Dolphin is a form Kirby and Prince Fluff can take on in Kirby's Epic Yarn. It an underwater-only transformation, and is only in a couple of levels. This form can only be taken on by jumping into a certain Metamortex.
When in Dolphin form, Kirby can swim very fast, considerably quicker than he can in Submarine form. He can get some serious air as well, jumping several feet out of the ground to get a treasure or beads. In this form he can also perform a surge attack (press the "2" button), spinning around and killing most enemies in his way. Kirby can swim up waterfalls easily in this form, and can balance balls on his nose to throw them into hoops.
The Dolphin form looks like a real dolphin, but with a dome on its head. Kirby is a pink and red dolphin with a blue tail fin, and purple dome. Prince Fluff is a blue and orange dolphin with blue tail fin, and a purple dome.