The point of this blog, thus, will be to answer all the questions - and fix the misunderstandings - that writers and games have about horses (and other equines - there will probably be some posts about donkeys/burros, mules, even zebras).
I've found there are two kinds of writers:
1. People who know and ride horses.
2. People who think they neigh when they run.
So...obviously I'm going to start by answering the title question:
Do horses neigh when they run?
In television and the movies, neighing sound effects often accompany chase scenes on horses the way squealing tires accompany those in cars. It's for effect and to make the scene more spectacular - but it makes any horse person flinch every time.
Horses cannot breathe through their mouths. When running they have to do everything possible to maximize air flow - and they extend their heads and necks in a position that opens the throat and the voicebox as much as possible. Because their voicebox is pulled to the side, horses not only do not neigh when running flat out - they actually can't.
Some horses do make a noise when running - a whistle or a roar when they breathe in. This is caused by damage to the nerves that control the voice box. For some reason, it's more common on the left side. In performance horses, surgery is often performed to permanently hold the paralyzed side in the open position - this affects the sound of the horse's voice but makes running much easier for them.
So, no. Horses do not neigh when they run.
(I am open to questions - any questions people might have about horses. A few I already have on the list: How are horses measured? Can you really ride a horse to death? How far can a fit horse go in a day without damage?)
(These cute guys are Icelandic horses...and they will be the topic of a post, most likely, at some point soon).