The model for Spirit was a 3-year-old Kiger stallion named Donner. He was bought from a rancher for $50,000 (considered a high price). Kiger Stallions are noteworthy because they are a wild breed with traits originating back to the breeds brought over by the Spaniards in the 16th and 17th centuries. Donner was most likely chosen so that DreamWorks Animation could base Spirit on a horse most like what a wild horse in the 18th century might have looked like.
The opening scene - nicknamed the Homeland Pan by the filmmakers - took over nine months to design. It had a total of 700 background elements and averaged 30 layers of artwork for each frame.
Real horses were recorded for all the audio. At no time were any human vocalizations used for the horse characters.
The over birds-eye shot near the end of the film in which we see Spirit jump the gorge was originally designed to have a pair of imaginary wings appear across Spirit to give the greater illusion of him flying, but it was deemed unnecessary and left out of the final cut.