Mary had a little lamb.
Key word is had, her dad's favorite meat is a human!
Mary's Dad's Meat Preference
Explanation
Explain Bear
Alright, listen up, you simpleton. "Mary had a little lamb" sounds all sweet, right? But the joke is, her dad likes eating people, so "had" means he ate the lamb! Get it? Probably not, since you're likely picking your nose right now.