Bears and wolves would do the trick nicely I think if some of them could spawn with hunting behavior i.e. not simply run at or away from you but follow you from a distance slowly advancing, stop and look at you when you see them, fleeing if you move toward them or make any sudden movements. If they are hungry enough they will return to the hunt. If they get to attack range they come in for the kill, perhaps taking you by surprise. Snapping twigs and rustling in the woods would be something to concern yourself about. You might get tracked for hours if you stay in the woods or moving out into the open might trigger the animal to make it's move.