You mean those Mayans who carved that "horse", or whatever large four-legged animal it was, didn't know what they were doing?
Or do you mean they carved that image while on peyote, and it was just some imagined animal?
As to the long-nosed Rain God, why is it the first European "archaeologist" (such as they were in the mid- 1800's) who drew the first drawings of the Mayan ruins with Chac's long curled nose, drew them clearly as elephants? (I think his name was Maudseley). A reasonable argument could be made that as one who had seen a living elephant those long curling snouts would remind him of an elephant. But why could not the same argument be made about the Mayans who carved those images? It is because the later PC Archaeologists all "knew" there were no elephants in the Americas--at least during the times the Mayans (or the much earlier Olmecs) populated Central America.
Conventional Wisdom often holds us back from the discovery of Truth that inconveniently contradicts that Conventional "Wisdom".
The Mayans wrote everything down - they wrote nothing about the horse or any large draft animal that had vanished within memory. Such a memory would have been around and have eliminated the shock of seeing Spanish mounted on horses.
Sorry, not credible.
PS: Are you a Mormon?
PS: The images of horses and elephants I am talking about are from carved stone slabs that were found in the mid to late 1800s. Most if not all were hoaxes.