North America's horses had been extirpated long before the rise to power of the Aztec civilization.
See the related Wikipedia link (Equus scotti) listed below for more information:
The Spaniards brang European men with diseases that the Aztecs were not naturally immuned to. The Spaniards also had armor and weapons that were made out of stronger material. The Spaniards had horses too.
horses
The Spaniards attacked the Aztecs during a festival
why did the spaniards treat the Aztecs harshly
The Spaniards were wanted to conquer the Aztecs and take all of their property, and the Aztecs didn't want that.
Small pox wass brought with the Spaniards which the Aztecs couldn't cure and, well, the Spaniards had cannons, horses and guns. (Instant ownage against clubs.)
They killed the Aztecs
the Spaniards defeated the Aztecs.
At first, the Aztecs thought that the Spaniards were gods because of their shiny armors and their weapons. When the Spaniards conquered them and onward, however, they feared them.
The Spaniards appeared to be nice to the Aztecs so they treated them nicely until Cortez betrayed the Aztecs.
the Spaniards
Montezuma-II