If they both were intelligent creatures, it seems pretty clear that there would be no moral justification in eating the rabbit, and the fox would be obligated to seek out another source of food.

Do the fox and rabbit get to call on human arbitration? Your argument might convince a rabbit, but I doubt a fox would buy it:

Foxes are far smarter and more beautiful than rabbits - and they depend on rabbits for their very existence. By contrast, rabbits simply mess up the countryside by covering it with burrows and droppings - they are a pest, and are obviously in need population control - though they seem too stupid to self-impose it.