What is the Difference Between a Filly and a Mare?

Learning any new language is challenging, and the world of horses is full of terms that can get confusing. There are yearlings, stallions, and colts, oh my! And those terms are only the beginning. We are also faced with the fact that not everyone in the horse world agrees on the exact definition for most equine terms. No sweat, though. You can still learn to speak the language.

The most effective way to get familiar with horse terms is to learn a couple at a time and get a full idea of what those terms mean. While you are studying them, learn the different ways they might be described by different people, so you will have a good understanding when reading or discussing horses with someone else.

This article is intended to help you get started. Two commonly confused horse terms are mare and filly, and knowing the difference is critical for anyone wanting to learn more about our equine friends. Start your educational journey on horses with the explanation of these words before moving onto others

Both fillies and mares are female horses. The difference is basically in the age of the two, similar to the way human females have different names for each stage of their lives. Fillies are young females horses, typically under the age of three, and mares are female horses over the age of three.

Fillies and Mares: A Female Horse’s Life Stages

Filly and mare refer to different stages of a female horse’s life. Just as human females move from baby to toddler to young child, and then adolescent, teenager, and adult, female horses have similar stages. The words filly and mare refer to two separate time periods a female horse experiences. She starts as a filly and then grows into a mare. There are some sub-stages in between, but understanding these two basic time periods helps clear up a lot of the confusion.

What is a Filly?

Many young animals, such as donkeys, mules, zebras, and ponies, are called fillies. Most often, though, a filly is a female horse that is three years old or younger. A young male horse is called a colt. While many people refer to all young horses as a colt, this is incorrect. A colt specifically refers to a baby boy horse. Though both are foals, girls are filly foals while boys are colt foals.

In some cases, a female horse is considered a filly until age four, as this is about the age they are done growing. Until the age of one, they are referred to as a foal or filly foal. Once weaned, which usually occurs around four to six months of age, they gain the name weanling filly. Some fillies continue to nurse until the next foal is born, though. Regardless of age, as long as the filly is nursing, it will be referred to as a suckling filly by most. Other people will simply refer to them as fillies.

At age one, they begin to be known as yearling fillies. At what point a horse becomes a yearling is one of those areas where the terms can get a little tricky. Most foals are considered to be one year old one year after the day they were born, just as we celebrate our birthdays. Thoroughbreds, on the other hand, are considered to be one year old on the first day of January, regardless of their actual month of birth. Therefore, the age of your filly depends on the breed.

Yearlings are not fully developed. Their bones and muscles are still weak at this time, so they are not quite ready to work as hard as grown horses. Only light training should occur at this point- if any. Allowing the filly to train alongside her mom will yield the best results.

Fillies are often used for riding, especially for children and beginners. They are less aggressive and have less will than colts, generally making them gentler and calmer to ride and learn on.

What is a Mare?

A mare is a female horse that is usually over three years old, but in Thoroughbred racing, mares are four years old and older. In some circles, a horse is not considered to be a mare until age five, though for most, age three is acceptable.

You may have heard some mares referred to as broodmares. These are mares that are intended for breeding, but they do not get this name until they have birthed at least one foal. In fact, the term mare is typically not used at all until the female horse carries a foal, no matter the age. 

Though fillies are sexually mature by the time they reach two years old, most owners do not breed them until about age four as it is safer for the mare and her young. This is just as human females begin to sexually mature long before most people feel their bodies are ready to carry and birth a baby. It’s not impossible, but parents of young girls and owners of mares both prefer for reproduction to wait until the bodies have fully developed.

Mares carry their foals, or are “in foal,” for about eleven months, so most broodmares give birth to one foal each year until the owner decides to stop breeding them or until they are in their 20s. It is possible for them to become pregnant with more than one foal, but it is rare. If she does become pregnant with twins, one or both of the foals usually dies as neither has enough birth weight to survive. Some twins do survive, but the chances of this happening are slim.

Mares have several different purposes all over the world. In some countries, mares are used for dairy and work purposes. Sometimes, they are used to compete in equestrian races. Other times, they are used just for breeding or riding. They may also be used for all or several of these purposes.

Mares make good horses for riding because, like their young, they are typically easier to handle. As female horses lack the presence of testosterone that colts and stallions have, you do not have to contend with those strong hormones on a regular basis.

However, mares can be moody when they are in heat, so don’t be surprised if your girl acts strange during this time. You can expect them to be in heat every 19 to 22 days, and you will know when she is as your mare will act nervous or harder to handle than usual. During this time, a mare may not be the best riding companion, especially if there is a stallion present. If not, the mare’s mood will typically be more of an inconvenience than anything else.

As every horse is different, your mare may or may not act any differently while in heat. If she seems to be overly aggressive, or if she is normally calm and has a sudden shift in personality, it is always best to consult with your vet. It may be nothing to be worried about, but some of these characteristics can point to serious issues. It is always better to be safe than sorry when it comes to your mare’s health.