A filly is a female horse that is too young to be called a mare. There are two specific definitions in use:
Fillies are sexually mature by two and are sometimes bred at that age, but generally, they should not be bred until they themselves have stopped growing, usually by four or five.  Some fillies may exhibit estrus as yearlings.
The equivalent term for a male is a colt. When horses of either sex are less than one year, they are referred to as foals.  Horses of either sex between one and two years old may be called yearlings.
A foal is an equine up to one year old; this term is used mainly for horses, but can be used for donkeys. More specific terms are colt for a male foal and filly for a female foal, and are used until the horse is three or four. When the foal is nursing from its dam (mother), it may also be called a "suckling". After it has been weaned from its dam, it may be called a "weanling". When a mare is pregnant, she is said to be "in foal". When the mare gives birth, she is "foaling", and the impending birth is usually stated as "to foal". A newborn horse is "foaled".
Lady's Secret was an American Eclipse Award winning Thoroughbred racemare that was listed in the Top 100 U.S. Racehorses of the 20th Century. Lady's Secret was bred by Robert H. Spreen at Lucas Farm in Oklahoma. Spreen sold her for $200,000 to Mr. and Mrs. Eugene Klein, and she was prepared for racing by Hall of Fame trainer D. Wayne Lukas. She was a small horse, weighing no more than about 900 pounds.
A mare is an adult female horse or other equine. In most cases, a mare is a female horse over the age of three, and a filly is a female horse three and younger. In Thoroughbred horse racing, a mare is defined as a female horse more than four years old. The word can also be used for other female equine animals, particularly mules and zebras, but a female donkey is usually called a "jenny". A broodmare is a mare used for breeding. A horse's female parent is known as its dam.
A.P. Indy was an American Thoroughbred racehorse who won the Belmont Stakes and Breeders' Cup Classic on his way to American Horse of the Year honors in 1992. His time in the Belmont Stakes tied Easy Goer for the second-fastest running in the history of the race, behind his damsire Secretariat.
Sky Beauty was a thoroughbred horse who won the 1993 Triple Tiara of Thoroughbred Racing. As of 2018, she was the last filly to win this title.
Azeri is an American Hall of Fame champion thoroughbred racehorse who was 2002 US Horse of the Year and Champion Older Female from 2002 to 2004.
Rags to Riches is a champion American Thoroughbred racehorse who won the 2007 Belmont Stakes, the first filly to win it in over a century.
Street Cry was a Thoroughbred racehorse, winner of the 2002 Dubai World Cup, the 2002 Stephen Foster Handicap and runner up in the 2002 Whitney Handicap. He was an international shuttle stallion that stood at the Darley Studs in Australia and the US.
Ginger Punch is an American Thoroughbred racehorse. Owned and bred by operations belonging to automotive parts magnate Frank Stronach, she is out of the mare Nappelon and sired by the Canadian-bred Awesome Again, winner of his country's 1997 Queen's Plate and the 1998 Breeders' Cup Classic and who, as a sire, has produced four Breeders' Cup winners including the 2004 World Champion, Ghostzapper.
Medaglia d'Oro is an American Thoroughbred racehorse who won several major stakes races including the 2002 Travers Stakes and the 2003 Whitney Handicap. He also finished second in the 2002 Belmont Stakes, the Breeders' Cup Classic in both 2002 and 2003, and the 2004 Dubai World Cup. Since retiring to stud, he has become an excellent stallion whose progeny include 2009 Horse of the Year Rachel Alexandra, two-time champion filly Songbird and two-time Hong Kong Horse of the Year Golden Sixty.
Kris S. was an American Thoroughbred race horse who is best known as a highly successful sire.
Zenyatta is a champion American Thoroughbred racehorse who won the Breeders' Cup Classic and Breeders' Cup Distaff and 19 of her 20 starts. She was the 2010 American Horse of the Year, and Champion Older Female in 2008, 2009 and 2010. She was inducted into the National Museum of Racing and Hall of Fame in 2016.
Stardom Bound is an American Thoroughbred racehorse who won the 2008 Breeders' Cup Juvenile Fillies and the Eclipse Award for Champion 2-Year-Old Filly.
Informed Decision is an American Thoroughbred racehorse who won the 2009 Breeders' Cup Filly & Mare Sprint on her way to being named the American Champion Female Sprint Horse.
Vertigineux was an American Thoroughbred racemare. She is out of the stakes-winning Forlì mare For The Flag, by the outstanding sire Kris S. Her owner is Eric Kronfeld, who bought back Vertigineux for $120,000 after she failed to meet her reserve at the 1996 Keenland yearling sale. Her name means "breathtaking" or "dazzling" (speed) in French.
Blame is a retired American champion Thoroughbred racehorse, a winner of nine races in 13 starts including the prestigious Breeders' Cup Classic.
Havre de Grace is an American Thoroughbred Racehorse who was 2011 American Horse of the Year. During her racing career, she was owned by Fox Hill Farms and trained by J. Larry Jones. Her sire is the 2005 American Horse of the Year, Saint Liam. Her dam is the mare Easter Bunnette, a daughter of Carson City. Her name derives from the city and racetrack of Havre de Grace, Maryland.
Beholder is an American Thoroughbred racehorse and multiple Eclipse Award winner. Her most memorable performance was defeating the champion filly Songbird by a nose in the 2016 Breeders' Cup Distaff. Her other notable wins include the 2012 Breeders' Cup Juvenile Fillies, the 2014 Breeders' Cup Distaff and the 2015 Pacific Classic against male horses. She is the first horse since 1976 to win a Grade One race at age 2, 3, 4, 5 and 6, and one of only two horses to have ever won three Breeders' Cup races. She was inducted into the National Museum of Racing and Hall of Fame in 2022, her first year of eligibility.
Cairn Rouge was an Irish Thoroughbred racehorse and broodmare. After showing promise as a two-year-old in 1979, Cairn Rouge improved to become one of the best three-year-old fillies in Europe in the following year. She won the Irish 1,000 Guineas and the Coronation Stakes against horses of her own age and sex, before defeating strong weight-for-age competition in the Champion Stakes. She failed to win in five starts as a four-year-old, but showed good form when finishing second in a controversial race for the Champion Stakes. After a brief, unsuccessful period racing in North America she was retired to stud, where she had some success as a broodmare.
Stellar Wind is an American Thoroughbred racehorse, known for her Eclipse Award winning three-year-old season, and later for her rivalry with the champion mare Beholder. Bred in Virginia, she raced in Maryland as a juvenile, winning one minor race from two starts. As a three-year-old, racing mainly in California, she improved to become arguably the best filly of her generation. In the spring of 2015, she won the Santa Ysabel Stakes and the Santa Anita Oaks before finishing a troubled fourth in the Kentucky Oaks. On her return to California, she recorded further victories in the Summertime Oaks and the Torrey Pines Stakes, and then ended her season by finishing a strong second in the Breeders' Cup Distaff. She was voted American Champion Three-Year-Old Filly of 2015.
It was the perfect end to a perfect day by a perfect filly. Zenyatta, the best older female horse in the country, completed a perfect season. . . by winning the $2 million Ladies' Classic on Friday at Santa Anita's Oak Tree meeting, the first of two days of the 25th Breeders' Cup championships.(Zenyatta was foaled in 2004, making her a 4-year-old at the time.)