Trappers are encountered in the wilds to the north of Riften at an unmarked location called Trapper's Dilemma, near a closed cage containing a wolf and baited with raw meat. They are apparently trapping wolves, and if they appear, they will be hostile to the Dragonborn. The wolf inside the cage (sometimes outside it, but the game believes it to be caged) is not hostile, and has enlarged eyes for some reason. The trappers will not appear (from behind a nearby rock) unless the Dragonborn opens the cage and releases the wolf. A dog may be encountered nearby that is also hostile, and may or may not belong to the trappers.
Once dead, the trappers are a lootable source of animal pelts, meat, hide armor, weapons, and other random loot.