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.
In the film, a real stallion named Giant's Causeway is mentioned. As well as being Europe's top racehorse, and one of the best stallions in America, his dam (mother) was Mariah's Storm, the horse on which this movie is based.
Kurt Russell filmed this simultaneously with Sky High (2005). He later said it was too taxing traveling back and forth between sets and different characters quickly, and probably would not allow himself to do that again.
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.
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.)
In the scene where Soñador is considered for the Breeder's Cup, Prince Sadir says that his horse "Rapid Cat is sired by Storm Cat, the best sire in the world." Mariah's Storm (on whom Soñador is based) was bred to Storm Cat.