Neptoona

Neptoona is the legendary fish of the ocean that can be found in Phantom Hourglass. She can only be found after receiving the Big Catch Lure and catching the Rusty Swordfish. The fish itself is a gargantuan, blue swordfish with a serrated bill with various green-blue markings. Showing the fish to the Old Wayfarer will result in Link receiving a Heart Container. Unlike other fish, her shadow on Link's map displays a shape of a swordfish.

Trivia

 * Even though it is a legendary fish, you can catch more than one.