From the moment of Christopher Columbus' discovery of lands previously unknown to Europe in 1492, the New World captured the imagination of European adventurers. Thousands of men came to the New World to seek fortune, glory, and land. For two centuries, these men explored the New World, conquering any native people they came across in the name of the King of Spain (and the hope of gold). They came to be known as the Conquistadors.
Conquistadors claimed that they were attacking the New World natives in order to spread Christianity and save the natives from damnation. Many of the conquistadors were, indeed, religious men, but history has shown that the conquistadors were far more interested in gold and loot.
From the moment that the first Horse was put ashore in November 1493, horses were the key to Spanish success in the Americas. They gave the Spanish not just distinct tactical and logistical advantages, but a moral advantage as well, because the native Americans had never seen such creatures before, and were initially scared to death of them.
At first the Aztecs thought that each cavalryman and his horse were one supernatural creature, with 4 legs, 2 arms and 2 heads! They were so relieved when they discovered that the horse was just an animal like any other that when the first one was killed , it was cut up and portions circulated throughout their lands, to demonstrate that such creatures were mortal.
It is therefore no wonder that horses captured by the Aztecs were sacrificed as if they were human, and their heads ended up alongside those of Spanish soldiers on the temple racks!