Final answer:
Hernán Cortés was the conquistador who profoundly influenced the Americas with the use of horses, starting with 16 horses in his expedition against the Aztec Empire.
Step-by-step explanation:
The conquistador who made a major impact with horses was Hernán Cortés. In 1519, Cortés brought 16 horses to the Americas, which gave the Spaniards a significant advantage against the primarily foot-soldier Aztec army. Over time, horses became integral to the Spanish conquest and colonization efforts, with Antonio de Mendoza, the first governor of New Spain, owning over 1,500 horses by 1547.