This will take quite some time as the legendary fish are very rare. After you invent the fishing pole, you can fish at any of the spots on the map marked with the fish icon for guaranteed fish to be caught, however they are random what you get. I got my fifth legendary fish to show up after exploring the map roughly 90% completed. Just keep at it and you will get it. I always swapped to the Tackle and eventually had the Fishing skills from level ups as well to increase fishing speed to make it less a hassle.