Era? 100+ years ago. It's mainly been in the last 100 years we've seen rules creep in to protect us from ourselves and others stupidity. I'm not talking rules governing crimes such as robbery, murder, etc.
As for hiking the AT and having a horse trip and fall hiking. This goes back to self reliance. If you are watching what is going on around you (back to self reliance) you would see that rider above you and try to maintain yourself in a place where if they fell they wouldn't land on you. FYI, a 200 pound hiker above you on a switch back can just as easily ruin your day as a 1000 pound horse. Heck, them just knocking off a 5 pound rock can ruin your day.