The movie was loosely inspired by the story of the mare Mariah's Storm. She was a promising filly who was being pointed towards the Breeder's Cup Juvenile Fillies in 1993 but then broke her cannon bone. She recovered and later won some graded races. She started in the 1995 Breeder's Cup Distaff and finished ninth. She was owned by Thunderhead Farms and trained by Don Von Hemel. She's now known mostly for being the dam of Giant's Causeway.
The role of Cale Crane was originally written for a boy. The role was changed specifically so that Dakota Fanning could play it. (The first script that was sent to Fanning actually had the word "boy" in the character description.)
When Cale and Ben go to Ashford Stud to check out the studs, the stallions they are naming, such as Fusaichi Pegasus, Giant's Causeway, Johannesburg and Grand Slam, are real horses who actually stand at Ashford. However, the actual stallions were not used in filming. Stand-ins were placed in their stalls instead.
In the film, a real stallion named Giant's Causeway is mentioned. As well as being Europe's top racehorses and one of the best stallions in America, his dam (mother) was Mariah's Storm, the horse on which this movie is based.