Neptoona

Neptoona is the legendary fish of the ocean that can be found in The Legend of Zelda: 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.