Mostly when the horse sees the owner going to the barn to feed them. Another time when a horse usually neighs is when they are comunicating. For example:
When a horse is left behind while any of the horses that he/she shares a pen/stall with leave for a ride. The horse left behind will try to communicate.
The horse let out a loud neigh as the farmer approached with its favorite treat.
Neigh!
The sounds a horse makes are usually written as a "neigh" or a "whinny".
Horses At a Horse Trial: "How does the defendant plead - Guilty or Not Guilty?" "Neigh!"
The word neigh is spelt as you have in the question.
neigh.
a neigh
a female horse
A horse can make a load of sounds: Snort, grunt, whinny, neigh and nicker. Also the younger the horse the more high pitched neigh it makes.
a neigh or whinny
neigh
Neigh!!!