I've never had a fox terrier but I've noticed that some breeds have some of the charateristics of vlcaks, primarily more primitive breeds. We had a litter of Finnish Spitz puppies a few years ago and they are VERY primitive. While the GSD puppies didn't need to be with each other all the time the finkie puppies were *always* in a pack and "hunted" the other dogs (and cats) in a pack. It was like Wild Kingdom in our house! The GSD and malinois didn't prepare us for the vlcak but the Finnish Spitz did (except for the finkie's independence).