Horses are useful for aiding travel in the New World. Scouts and Dragoons move significantly faster than do colonists on foot. Any colonist may be mounted to create scouts and any soldier can be mounted to create dragoons.
It is best to have horses in each of your colonies to provide a quick mode of transit when need be, give your soldiers a boost, and a decent resource to trade with the natives.
|Sid Meier's Colonization Goods|