Final answer:
The Spaniards' introduction of the horse had the most lasting impact on the Plains tribes by transforming them into efficient hunters and nomadic warrior societies.
Step-by-step explanation:
The Spaniards left the most lasting impact on the Plains tribes through the introduction of the horse. The reintroduction of the horse at European contact significantly transformed the culture and lifestyle of many Plains peoples. The nomadic Plains Indians became efficient hunters and warriors, utilizing horses to hunt bison herds and gain a tactical advantage over their enemies. This shift also allowed groups to transform from farmers to nomadic hunter-gatherers, influencing their social structure, warfare, and the centrality of the bison in their religious systems.