Answer:
A cannon and soldiers on horseback.
Step-by-step explanation:
From the description given by the Aztecs, it is likely that what they are seeing for the first time is a cannon and soldiers riding on horseback.
The Aztecs described it as "a thing that a ball of stone comes out of its entrails" and if it were aimed at a tree, "it shatters the tree into splinters", almost as if "the tree had exploded from within" which is a very good description of a cannon.
Also, they described something about men "dressing in iron" and their "deer carrying them" anywhere they wanted to go, which is a good description of armor-wearing soldiers on horseback.