What is a three-legged thing flying in the sky?

"Professors and farmers sit in the same car. The professor suggested, "Let's play a game and ask each other questions. If you can't answer, you will lose one yuan each. " The farmer shook his head: "No, you are a man of culture. The professor flinched and said, "well, I'll pay you one yuan and you pay me fifty cents." The farmer nodded in agreement. First of all, he asked, "What is a three-legged thing flying in the sky?" The professor thought hard and finally paid one yuan to ask. The farmer replied, "I don't know, so I ask you." "A pay professor fifty cents, earn fifty cents. This move is suspected of cunning, but we must not ignore the creativity of farmers. "

I guess it should be "three-legged bird" and "three-legged worm", just like hydra and five-legged cow. The world is big and wonders.