I agree your first photo does have the appearance of a snake.
It is always good to have a creative imaginative mind, but unfortunately I would say this is a naturally formed rock.
The Native American artist were very talented. If they carved a snake on a rock and hundreds of years later you found it, you would not have to guess what it was. You Would See a SNAKE!! Look on Google or elsewhere for Native American artifacts and you will see what to look for in the future. Good luck and happy hunting!