Animals appear everywhere (in particular birds), and I've found hares and even elks spawned on single tile islands, so foxes can presumably be spawned there as well, but it wouldn't be very frequently.
It can also be noted that trap fences do not work well with small game traps, as small game can simply run through the fence (small here <= lynx). Wolves can pass fences (jumping?) but tend to go through the trapped openings (less work?).