Complex sentence: More horses were added to the population that would become the Marsh Tacky through horses that were purchased in the Spanish settlement of St. Augustine in Florida, used as pack horses on Native American trade routes, and sold when the traders reached Charleston.
Split into simpler sentences: More horses were added to the population that would become the Marsh Tacky through animals that were purchased in the Spanish settlement of St. Augustine in Florida. They were then used as pack horses on Native American trade routes, and sold when the traders reached Charleston.