Foxes hunt small animals and scavenge off larger things or whatever's convenient. Garbage or at camp sites and whatnot. They're in the canine family, but they're not like wolves or anything. But they would be hunted by wolves and coyotes. So that's out. Any larger herbivore would be too big for the fox to hunt, and probably wouldn't consider the fox much of a threat. So in that regard I'd say a deer or moose or something

On a side note, if the fox happens to be a pet, its interesting to note that they can easily be domesticated if not tamed. This has been done many times in the past (its even more interesting to note that the more generations of domesticated foxes come by, the more their attributes take on that of dogs; their fur changes and lightens, they sometimes start to develops patches or spots like hounds or dalmatian mixes). But like dogs, in the proper environment, they can be friends with anything