Infertility.
Because the stallion is a male and the mare is a female.
The child of a mare is called a foal.
The mother of a mare is called the dam.
You're right all mares can have foals. But not every owner whats their mare to have foals. Mares that have owners that what them to have foals are called broodmares. Mares that are not going to be used for breeding purposes are just called mares.
A female horse is called a Mare baby horse is called a Filly
A mare is a full grown female horse. An adult mare is called a mare. It is sometimes used as a euphemism for a married woman.
A stallion's mate is typically called a mare.
A female horse is called a mare.
A Mare or a Filly
A mare that has a foal is called a broodmare.
Its called a dam, the father is called a sire.