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12 Fun Facts About Filly Horses

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If you are relatively new to the horse world, you may be a little unfamiliar with some of the terminology, especially terms that apply to horses of specific ages and gender. For example, you may have heard the term filly in reference to people and horses, but do you know exactly what it means in the horse world?

Young horse resting on the green meadow

The term filly describes a horse of a specific age range and gender. In general, a filly is a female horse under the age of 4. In some parts of the equine world, female horses under 5 years of age are called fillies. As a filly, the horse will experience a number of vital changes as it matures into a mare.

Learning all the terminology associated with horses can seem overwhelming at times. Thankfully, just knowing some key information about fillies can help you feel a little more prepared for the journey ahead. Fillies are not just female horses as many may think; more specifically, they are female horses experiencing a vital and life-altering stage of their lives.  

1. A Filly is a Female Horse Younger Than 4 Years Old

In general, filly horse is a female horse that is younger than 4 years old. More specifically, when used alone, the term filly often describes a female horse from 2 years old until it reaches the age of 4.

Many equestrians will call any young female horse a filly, regardless of its exact age, and they are not really wrong. Once a female horse reaches age 4, they are should be referred to as a mare instead of a filly. (source)

2. The Plural of Filly is Fillies

The plural form of the word filly is fillies. The word filly is derived from the Middle English word fyly and was first used in the 15th century. (source)

3. A Filly Cannot be a Colt

No, a filly cannot be a colt. The term colt is the male equivalent of the word filly; therefore, colt can only be used to describe a young male horse under the age of 4.

There are rare times when a foal is born with both male and female parts as well as hormones. These horses are often referred to as intersex and may appear to be a filly, but hormonal tests may indicate that it is genetically considered to be a colt! (source)

4. A Foal Can be a Filly

Technically, a female foal can be referred to as a filly foal when it is first born. It will be classified as a foal or filly foal up until it reaches 12 months of age.

A mature horse and a foal beside it grazing

From age 1 until it is 2 years old, the horse is called a yearling or a yearling filly. Once it reaches its second birthday, a female horse then becomes known as simply a filly.

Some equestrians will reference the horse’s specific age during that time, such as calling it a two-year-old filly or a three-year-old filly.

5. Racehorse Fillies Follow Different Rules

The racehorse world has different standards for what age constitutes a filly. While they agree that a filly is a female horse, they disagree on the technical cutoff age for classification purposes.

For racehorses, the cutoff age for a filly is 5, not 4. Until they celebrate their 5th birthday, they are called fillies. When a female racehorse reaches the age of 5, they are henceforth referred to as mares.

6. Fillies Become Mares

A filly becomes a mare when it reaches the age of 4. In the horse racing world, a filly does not become a mare until it reaches the age of 5.

Around the ages of 4 and 5, horses are no longer considered to be young and are very close to being full grown, hence the final name change.

7. A Weanling Can Be a Filly

A weanling horse can be a filly or a colt. Weanlings are foals that have been weaned off their mother’s milk or off the bottle if they have been bottle fed. They can survive solely off water and solid foods like hay, grain and grass.

Foals are usually weaned around 6 months old. Many horse owners will call their weanling a filly weanling if it is a female. If it is a boy, it will be referred to as a weanling colt.

8. A Yearling Can Be A Filly

horses graze on the farm field

A yearling can also be a filly. Female yearlings are often referred to as yearling fillies. In this case, the filly term is used to specify that is horse is female while the word yearling identifies the horse’s specific age. A male yearling can be referred to as a yearling colt.

9. A Filly Can Go Into Heat

A filly can go into heat before its ages into a mare and if it is healthy, it probably will. If it does not, there may be a reproductive issue at play. In some instances, a filly can experience its first heat at 14 months old.

Typically, a female horse will experience their first estrus cycle at around 18 months old, which means they will still be considered a filly when they first go into heat. (source)

10. A Yearling Filly Can Become Pregnant

A yearling filly is a female horse that is between the ages of 1 and 2. Since a female horse can technically become pregnant as early as 14-months old, it is possible for a yearling filly to become pregnant if exposed to an intact male horse.

11. Young Fillies Should Not Carry Foals

While a horse can get pregnant while it is still a filly, that does not mean that it should. Just because a filly has a heat cycle, it is still not fully grown.

A filly can have issues carrying a foal and it could even hinder their overall growth.

The nutrients that would normally go to the filly will be rerouted to the growing foal instead. (source) With proper nutrition and close care, a healthy filly can carry and deliver a healthy foal, but they should be closely monitored throughout the pregnancy.

12. You Can Start Training Your Filly Early

Mother wild horse with baby foal in field along Salt River in Mesa, Arizona

You can begin training your horse while it is still a filly. Every horse is different, and a seasoned trainer will recognize when training can begin. Most equestrians can introduce a saddle to a filly by the time the horse is 3 years old, sometimes even as young as 2 depending on the horse’s disposition.

Of course, carrying a rider is a whole different story. More and more people are letting horses mature to four or even five years old before adding a rider.

Halter training, lead training, and ground manners can all be successfully taught while a horse is still a filly. It is important to note that racehorses often begin training as yearlings and are already racing at 2 or 3-years-old.

Final Thoughts

Fillies are female horses under the age of 4 in the general equine world, or under the age of 5 in the racehorse world. They progress from a foal to a weanling to a yearling and then into a 2-year-old and eventually a 3-year-old filly. Fillies will experience their first few heats, their first time wearing a halter, and will often begin their training to become a horse that can eventually be ridden.