Chullin 79: Horses, Donkeys, Mares and Hinnies

Our Mishna teaches us about mating different species of animals (from Vayikra 19:19).  Rabbi Yehuda tells us that if a mule is in heat, it can be mated with another mule but not with a horse or donkey.  The Gemara teaches that all kinds of horses and donkeys are forbidden because we do not know the mule's actual species.  To learn its kind, we use the signs taught by Abaye:

• if its voice is deep then it is the offspring of a female donkey
• if its voice is shrill then it is the offspring of a mare

Rav Pappa adds to this:

• if its ears are large and its tail is small it is the offspring of a female donkey
• if its ears are small and its tail is large it is the offspring of a mare

Rabbi Yehuda does not consider the consideration of the physical traits listed.  The Gemara asserts that we must be dealing with a case where an animal is mute and it had its ears and tail cut off thus it could not be examined.

If a mother horse mates with a donkey, the offspring will be a mule.  Mules can be male or female and are infertile.  If the mother donkey mates with a horse, the offspring is a hinny, which is smaller and less common than mules.  Steinsaltz shares detailed information about these different animals including their appearance and their characteristics.