Here is a photo of the offending animal, which the man and I now believe to be a fossa. For those of you who are unaware, a fossa (pronounced foo-sa) is a mammal that looks very much like a cross between a dog and a cat - or, a cross between, perhaps, a dingo and a bobcat (such as our Jack).
Since we found Jack in a ditch, it is difficult to know his exact origins.
I take Jack to the vet on Friday for his second distemper shot. Perhaps now that he is older it will be more apparent that we are harboring a creature once thought to only inhabit Madagascar.