The story is about particular animals. I don't know this site as well as I should, so can't check. But in my memory of the story (published originally in Aesop's Fables) it starts 'A crow was trying to [I forget what] when a fox saw it and...' or something like that - bringinging the focus down to a particular pair of animals.
As the story starts with indefinite articles, we are brought from the general to the particular: "A fox and a crow were...." (OK, the story's about these two animals, so from now on...) "the fox said...' and so on.
So, the phrase 'a fox and a crow' can sometimes be used; but the title of a story has to use definite articles. (In something that isn't a story - say, a scholarly discussion - indefinite articles would be possible: 'Comparison of the responses of a fox and a crow to global warming; a study of ...' [this would be a study based on just two animals - pretty unlikely ]