I doubt it. We have a mountain of evidence, including archeological, anatomical, written and oral traditions saying that we started riding horses somewhere between 3000 and 2000 years ago, and we have this suggesting it was 5000 years ago. We have bones of horses showing that 3000 years ago they were too small to support an adult human rider, we have paintings and descriptions from 3000 years ago showing horses being smaller (e.g. from Egypt), be we have oral traditions saying humans used to ride chariots dragged by a pair of horses (e.g. from Homer), etc. All the "horse riders" would be risking death, simply because horses were too small, the weight of rider would break the horses back and throw the rider down and kill or injure him (and injury was death).