Mules are the offspring of a female horse and a male donkey. A good way to remember the difference between a mule and a hinny is to remember the phrase, 'Mule, mother, mare'. A mule winds up with 63 chromosomes, just as a hinny does, because donkeys have 62 and horses have 64. Although the odd number of chromosomes generally makes mules sterile, there have been cases where they have been able to reproduce. Mules tend to have a horse-like body, ears that fall somewhere in between the length of a donkey and a horse, a fuller tail and a bray/neigh...quite an interesting sound if you are lucky enough to hear it. Compared to their donkey father, mules tend to be much stronger, athletic and have quite a bit more endurance. They are able to withstand dryer, hotter conditions than horses and tend to eat less than horses in general.

You can see the 'donkey cross' on Terra's back, as well as her very horse-like tail.