To catch the Hylian Loach, go to the northern-most cove during the summer. Cast the Frog or Sinking Lure in the lily pads. Once he bites, he is hard to reel-in.

In the Zora Temple, find the room where you get the Boss key. There will be Skull Fish and two Hylian Loaches. Be careful not to catch a Skull Fish as they just take up bait.