Complex sentence: East Prussian farmers were encouraged to bring their mares, by then known for their hardness and quality, to Trakehnen's stallions, which allowed for the rapid transformation of the breed into much sought after army remounts; sure-footed, intelligent, and athletic.
Split into simpler sentences: East Prussian farmers were encouraged to bring their mares, by then known for their hardness and quality, to Trakehnen's stallions. This enabled the rapid transformation of the breed into much sought-after army remounts: sure-footed, intelligent and athletic.