How did the horse affect the conquest of Mexico?

Answer 1

While the Aztec and the Maya were extremely advanced civilizations with powerful warrior cultures, the fact remains they had never seen a horse before.

When the Spanish became hostile to the Aztec, the Aztec thought the Spanish knights in their armor on warhorses (also armored) were fire breathing dragons, and/or demons! This had an incredibly potent psychological effect on the Aztec warriors.

While the Aztec and other Mesoamerican civilization warrior cultures were very deadly with the axe, atlatl, spear, and other weapons, they'd never seen a gun before, either. Combine trained Spanish soldiers with trained horses, attacking war dogs, guns, armor, and horribly fatal diseases that the indigenous immune systems were unprepared for, and you have the beginning of the end of the Aztec Empire and eventually the Maya. (Although the classical Maya city states fought each other often and other mysterious factors led to their eventual decline and disappearance.)