Red-tailed catfish are a type of fish.
Like all types of fish, each red-tailed catfish gives one fish fillet when caught.
Catching red-tailed catfish
Red-tailed catfish can be caught with red and green lures in swampy spots only - these are fishing spot #9 (the swampy area next to the Duck Salon), spot #13 (the swampy area in the bottom left corner), or in spot #14 (the swampy area at the bottom, below spot #11) - and can be caught only during fishing events. When fishing in those spots under those conditions, if a small circle appears when using a red lure it is guaranteed that it will be a Red-tailed catfish, as it is the only fish that can be caught by using a red lure with a small circle.
It should be noted that the probability of hooking a red-tailed catfish with a red lure seems to be smaller than when using a green lure - perhaps in the neighborhood of a 2% to 5% chance on any given cast. It often takes dozens of tries with a red lure before a small circle appears, indicating a red-tailed catfish.
As is the case with other trophy-sized fish, red-tailed catfish are active, often pull hard and unpredictably, and combined with the smaller fishing circle these traits make them harder to catch than most fish.
See Fishing List for more detailed information.
Red-tailed catfish in the fishing book
The red-tailed catfish is on page 19 of the fishing book:
This big, beautiful fish has a massive appetite. You might lose a lure or two! It's the only seasonal fish which can be caught with a red lure.