Sooo...I just caught it finally last night...it is in the Vesperpoo, andl you will need to first get plenty of Dragon's Beard fishing line, the reel and rod from the shop next to that spot (which are like 18000 gil combined), and a green lure which I'll be honest I don't remember the name of and I'm nowhere near my game at the moment, sorry...but you may also want to be at level 7 skill level for fishing as well, since you get linger lasting line.
The key is you need to camp at the haven near that, and when you do there will be a cut scene where Gladio will ask you in the middle of the night like a weirdo if you wanna wake up at the ass crack of dawn to catch that legendary fish...it then becomes a tour, and i believe you will need to also clear the entire vesperpool before it will engage you...take note, it doesn't appear on the HUD, and you will simply have to keep casting til it takes interest in your lure, and then entice it to bite...
After that, once you've got him hooked, you really really REALLY need to be patient...it took me about 4 tries, and I had been fishing for almost 3 hrs to get my skill up, so I was kind of in a groove by that point...his stamina is off the charts and you can only reel when he stops struggling, so you'll need to pay special attention to line tension and also catch the jumps to, if you know what I mean...his "attack" will deplete your line very quickly if you don't just go with the flow and take your time...get ready for 20 minutes of battle fishing, and good luck!
Ps-I tried to upload the photo I took so you could see what your catch would be but it wasn't working...it's gratifying if you like a challenge though
There is a degree of trial and error I've found.
Most of the lures will tell you what type of fish they work best with. So you can couple this with the map to get an idea of what you should be using.
So if you're in Cleign and you have a lure for Cleign Bass for example, safe bet that's a good place to start. Plus if you're fishing in the ocean, try lures which attract 'Sea Bass' and similar.
Also, it's worth scouting the local area, quite often there is fishing tackle laying around, you can pick up, which also helps give you a clue as to which lure to try first.
However it's worth noting that you can catch fish with the 'incorrect' lure, it's just harder to get a bite.
"Perhaps you should respool your line Noct?"
Or check this guide for some extra information. I've quoted the relevant section below.
Lures : There’s lots of lures in the game. You’ll need to swap out which lure you use based on what fish you’re hoping to hook and where. Popper lures let you seek fish near the surface, pencil lures let you fish more effectively at dawn or dusk, swisher lures make boring fishing spots more lively, crankbait lures help you in rocky or littered waters, minnow lures work in clear waters, and vibration lures are best for wide areas.
Lure Weight : The heavier the lure, the deeper underwater it goes, and the bigger the fish it catches.
Lure Color : Fish like colors close to their own. Get a red lure for a red fish.
There are many lure possibilities, but pick the one best for the environment you’re hoping to catch the fish.
Lines: With only three types of line, you’ll mostly be purchasing these from shops. The higher the durability, the longer it can hold out against tension (aka damage) inflicted by fish.
Rods: Rods provide defense, which determines how much tension damage on your line they cancel out. You can get better rods from shops and by completing sidequests for Nayth.
Reels: Reels determine how fast you knock out a fish’s stamina bar as you reel it in - basically your fish attack stat. Tackle shops have high-quality reels, but the very best reel is called Llymlaen and is a reward in the Totomostro mini-game.
Best Answer
From my experience with fishing in FFXV the way you entice the fish is by doing short bursts of reeling in while moving the lure from left to right to get it moving. Once the fish bites make sure you get the QTE to hook the fish so you can start the reeling in process.
After you enter the reeling in phase slowly start to reel in the fish if you start to see the red and white flashing bar stop reeling in order to keep your line from snapping. If the bar tells you to move your left stick to the left or right make sure you do as such since it builds tension a lot if you fail to move your line. Also be sure to pay attention if the fish decides to jump in the water since it will prompt you for another QTE to keep the fish on the line. Repeat until you catch that fish!
And make sure you feed that cat!