Seahorse

From Zelda Wiki.org

The seahorse is a character found in Majora's Mask. To free the Seahorse from the Fisherman at Great Bay, a good picture of one of the Gerudo Pirates is required, which can be easily obtained by equipping the Stone Mask and photographing any one of the Pirates in the compound.

After Link gives the photo to the fisherman, he will receive the Seahorse in exchange, which he will then take to Pinnacle Rock to release. The seahorse will then guide Link through the misty waters to where the Sea Snakes dwell, trapping three Zora Eggs as well as another seahorse. Once Link has killed each and every one of the sea serpents, the other seahorse will be released. Both will thank Link for his help, and will reward him with a Piece of Heart.

The seahorse is described by the fisherman as a rare specimen.