Depending on how loose you are with the geographic location of animals, the replacement for the heavy armor characters could be elephant riders (or possibly a similar but mythological creature but none are coming to mind) I kind you not, I google'ed "Do elephants live on mountains" and the answer I got was "They would fall" though a quick look at Wikipedia said nothing about them living on mountains so I presume they don't, but.. elephant riders. The elephants don't even nessicarily need to be local, they could be brought over by trade or hunters. Really, they still will be spearman and bowmen, but as trade of for being grounded, I presume elephants aren't the easiest things in the world to kill. (if you don't have an elephant gun)