some spawners are generic; eg if you kill the necks you get deer, kill the deer and get boar... etc. Some seem to be more focused, like right outside a troll cave.
try killing everything that moves in your forest patches and it could trigger one, but this is an ongoing counter to the 'just get 50 troll hides instead of metal' threads. About 1/2 the new worlds I have started have not had enough trolls to make a full set of upgraded armor in a reasonable timeframe.