I noticed there was no guide on the twilight snapper, even a guide for all elites (and more specifically a guide for the putrid rat).
How to Get Twilight Snapper
To get a Twilight Snapper make a fishing rod in the woodworking bench after you kill Rufus the Foreman the rod and workbench unlock.
You can to find them fishing in Farbane woods, more specifically it seems to be a drop in the top left near Silverlight Hills.
What to Do with the Fish
You can build a crafting station called Vermin Nest. You need this fish to summon the elite rat. You can learn the rat form from it.
A friend of mine claims he just found the fish in a Chest in the Copper mines. Haven’t seen it myself yet tho