Hylian Loach

The Hylian Loach is a large, rare fish that appears in both Ocarina of Time and Twilight Princess. The loach looks like a cross between the catfish and the pike, with an eel-like body and ornate swirl patterns down the length of its body. They generally weigh in at about 29-36 pounds and can only be found at the Fishing Holes in its respective game. Due to its rarity, the Hylian Loach is usually revered to be a legendary fish.

Ocarina of Time
In Ocarina of Time, the loach appears purely by random at the fishing pond in Lake Hylia. The best way to find it is by equipping the Zora Tunic and the Iron Boots, and then entering the water and scanning the sand floor and the area around the lily pads. If it can't be found the first time, Link can just exit the area, enter again, and repeat the process. They can be caught with either the regular lure or the Sinking Lure, although the loach will only bite onto the hook as it goes up to the surface.

When Link takes the loach to the pond owner, he will tell Link that the fish population's number has decreased and release it instead of keeping it in the fish tank. He will also give the young hero a reward of 50 Rupees.

Twilight Princess
In Twilight Princess, Hylian Loaches can only be found at Hena's Fishing Hole in the summer time around the lilypads. They can only be reeled in by using the Frog Lure or the Sinking Lure. "Baby" Hylian loaches can also be caught by using Link's fishing rod while bobber fishing. Despite that Hylian Loaches are more common by now, they are apparently still legendary, and Hena is remarkably impressed when Link catches one.

The Legend of the Hylian Loach: Twilight Fish
A book called The Legend of the Hylian Loach: Twilight Fish can be found in Hena's fishing hut. It gives a description of the behavior and habitat of the loach, mentioning that they prefer the shallow waters and love frogs; moreover, the book explains that they are known to be "ansty" and prefer the summertime over wintertime. According to Hena, who has read the book many times to the point of memorization, the book is about a young Hylian Loach and its quest to save Hyrule.

Phantom Hourglass
In Phantom Hourglass, a portrait of the Hylian Loach can be seen in the middle of the Old Wayfarer's house on Bannan Island.

Spirit Tracks
In Spirit Tracks, the Hylian Loach portrait from Phantom Hourglass also appears in a house in the north-eastern corner of Hyrule Castle Town.

Trivia

 * The loach also makes a small cameo in Animal Crossing: when a loach is caught, a message appears saying, "You don't suppose it's Hylian, do you?".