The former is the title of a video, https://www.youtube.com/watch?v=sXTmdWcfXUw
I am wondering why the title is not 'A Girl and a Wolf'.
I have not yet watched the video, so I do not know which girl and which wolf the author is referring to.
If listeners can be assumed to be familiar with some relevant relationship, or story, or parable about a girl and a wolf, it would be "the girl and the wolf". Otherwise, if that cannot or should not be assumed, "a girl and a wolf".
While the "familiar to the listener" vs. "unfamiliar to the listener" criterion is useful, it is not the only thing that goes into whether the definite article can acceptably be used.
In the title of a story, a definite article is often used before a nominal that refers to some specific person or thing in the story. E.g. "The Old Man and the Sea", "The Great Gatsby", "The Brothers Karamazov", "The Trial", "The Invisible Man", "The Idiot", "The Tin Drum", "The Little Prince". (Note that some of these are translated from other languages, such as French or German—which do have definite articles, and which seem to share with English a similar convention of using them in titles like this—and Russian, which does not have a definite article.) I would say it is best for an English language learner to just remember that the definite article can be used this way; I can't think of any easy way to explain it by consciously applying some principle about the use of the definite article.
The reader is not necessarily expected to know who or what the definite noun phrase is before reading/hearing/seeing the story. I think we can say that the definite article is appropriate because the noun phrase is definite in the context of the story itself.