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 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 spot at the bottom, below spots #11 and #10), and only during fishing events. When fishing for them (in those spots only) 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 2% to 5%. It often takes dozens of tries with a red lure before a small circle appears, indicating a red-tailed catfish.
Red-tailed catfish can be caught in medium and small circles and therefore are harder to catch.
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.