My horse has seen it all - we've had to pass close by (and behind) all kinds of vehicles and farm machinery and he never blinks. We've had ATVs and dirtbikes go by while we're on the trail, we've seen hikers and cyclists and loose dogs and peacocks and geese, various wildlife. . .he doesn't care.
The biggest reaction I ever got out of him was one Halloween, when I was out riding while the nearby park did a special trick-or-treat event in the campground. We were crossing a road and a tiny little girl, no more than two or three years old, was waddling around dressed as a pink Care Bear.
My horse came unglued. . .prancing and snorting and staring like
"WHAT is THAT??"
It was several years ago, though, and I think he has managed to get over his Care Bear phobia.
The other thing I discovered he doesn't like is trail wagons pulled by a team of mules or horses. We ran into a whole freakin' caravan of them at Land Between the Lakes in KY and my horse was not
amused. I don't think the wagons or teams bothered him, because he saw them around the campground and seemed unphased. I think he was more bothered by the fact that we got stuck in amongst the caravan and could not get around them, and had to keep stopping to wait while the wagons went down steeper sections of the trail.
Finally, the trail broke off into a smaller side-path and we took that to get around them. To this day, though, my SO and riding buddies joke that there is a wagon-sized hole in my horse's training.