Question: how do you catch the legendary fish on red dead redemption 2?
Answer: Accept the Stranger Mission 'A Fisher of Fish' (available in Chapter 3).
Buy Special Lures from the Bait Shop in the Bayou (only available after accepting the above quest)
Go to the Legendary Fishing Spot (see guide below)
Equip the correct lure for the water type (River, Lake or Swamp).