Going to the very basics now. What do we call male and female horses? How about donkeys?
A fully grown, adult female horse is a "mare." A young female horse is a "filly" - from the French "fille" for "little girl."
An entire, adult male horse is a "stallion." You may also hear him called a "stud" in some regions, but in other regions that term is considered entirely incorrect, so stallion is safer. A young male horse is a "colt."
One point of confusion is that in the west all young horses are "colts" with males being called "horse colts" or "stud colts" and females being called "filly colts" or just "fillies." In any case, a field full of young horses is a field full of "colts" (or "youngstock").
A male donkey is a "jack". A female donkey is a "jenny". Donkeys as a group, though, are sometimes called "jacks" or "jackasses" and some donkey breeds contain the word "jack."
A castrated male horse or donkey is a "gelding" regardless of age.
A male mule is a "john" and a female is a "molly."
Confused yet? A lot of horse terminology carries a weight of tradition with it - and there are some regional variations that even those experienced with it can find a little strange.
An Appaloosa mare.