Horses don't sound like that.
I'm watching the movie Outlander. These Vikings have got to have the most vocal horses I have ever seen! They won't stop whinnying or sneezing or neighing or snorting or wuffling. The sound guy or producer act like they have no idea how quiet horse herds actually are.
It's an innate survival skill, just like how no one told us to be quiet when playing hide and seek. Those ancient brain spots know that the most noisiest creature is the first to be found. Because back then, not being found meant you could hide to live another day, or attack prey and live for another day.
And that's when realized...
Tl;DR - Why do so many movies with one or more horses make them sound so noisy? Horses aren't that noise -alone or in a herd.