The legendary fish in Stardew Valley can only be found in specific locations, and some require high fishing levels in order to catch.
Stardew Valley legendary fish locations
The ultimate Stardew Valley fishing guide - Android Authority
Year 1, Day 22, Legend Catch! : r/StardewValley
Stardew Valley Expanded: New Fish Locations
PSA: In the new Extended Family special order, where you have to catch the Legendary Fish again, you can use Wild Bait for a chance to double catch the legendary fish
How and where to catch all Legendary Fish : r/StardewValley
Sewers - Stardew Valley Guide - IGN
Stardew Valley - Legendary Fish; How to Catch & Locations
Stardew Valley Fish Guide
How to Catch the Legendaries
Stardew Valley: 10 Best Fish (And How To Catch Them)
The ultimate Stardew Valley fishing guide - Android Authority