Mule

The classic hybrid: a male donkey crossed with a female horse. A mule has 63 chromosomes, 32 from the horse and 31 from the donkey, and that odd number makes it almost always infertile, though rare pregnancies have occurred naturally. People keep breeding them because the cross works. Mules are more patient, hardier and longer-lived than horses, and they are still everyday working animals: Mexico had the world's largest mule population in 2018, at more than 3.2 million.