Mules are the offspring of a female horse and a male donkey. But mules generally can’t reproduce on their own. The ancient Romans even had a saying that commented on this, saying “When a mule foals!” similar to how we’d say “When pigs fly!”
So what’s up with mules and reproduction? What is it about them that makes it so that mule-plus-mule reproduction doesn’t work? Long story short, there are mismatches in horse and donkey DNA. For one thing, horses have 64 chromosomes, while donkeys have just 62.
The weird thing is that, in extremely rare cases (such as this one in 2007), female mules have managed to generate functional eggs that do allow mule reproduction. Scientists are not clear on how this works, but it's basically an example of pigs flying.