How can we tell if there were spotted horses 1000s of years ago except through pictures> I'd believe the pictures known to be of that age before I'd believe that there were none. What about humans? Do you think that there were blue eyed Neaderthals? Green eyed cats? Blue eyed wolves?
Well you can’t prove a negative but there is evidence that it took a lot of breeding to get spots on horses such as we have in the modern day. If other parts of the paintings are abstracted - and they appear to be unless those horses were also so thick necked as to have difficulty eating and moving - it doesn’t seem entirely unreasonable to think that coat colors might have been embellished a bit. Even if only to look beautiful.