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 could base Spirit on a horse most like what a wild horse in the 18th century might have looked like.
Real horses were recorded for all the audio. At no time were any human vocalizations used for the horse characters.
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.
During the opening and closing sequences, the cloud formation looks like a running herd of horses. Also, when Spirit is chasing the eagle early on a head-on view makes Spirit appear to have the eagle's wings, and then the same happens later when there's a shot of the eagle's shadow over Spirit.