Since Gnarwails were added they only dropped their horn once on either it being taken off when the gnarwail impales your boat, or when the gnarwail dies. I found when the horn was added it was fine for it to drop once as it had more minimal use in the ocean, and keeping the horn on the gnarwail was more useful as it could toss you hard to obtain salt in the brine when befriended.
But now in the addition of the strident trident which not only requires one gnarwail horn, but three and a kill from a boss which has at least 3 times more health than malbatross makes it a bit hard to obtain. It usually takes me 7-15 days to find one gnarwail when fishing with it's %5 chance to spawn when new shoals of fish spawn in deeper ocean. Which means to make one trident it would take me about 21-45 days to get the materials, which makes it less useful and to me more of a trophy item of my hard work.
Now my suggestion about getting horns is giving gnarwails the chance to regrow their horns as it seems like they are covered in them. When gnarwails lose their horns to you, they lose their ability to fish, so maybe if you give them 5 big fish or 10 little fish and wait a few days they could regrow their horns? I don't know what would be the best way for allowing them to regrow their horns or possibly obtaining them elsewhere from The hermit.