Horses being atypically vocal.
Horse comes to a stop: Snort.
Horse gets scared: Repeated and long neighing.
Horse is seen for five seconds, standing still and doing literally nothing: Snorting, nickering, and neighing.
In general, a horse snorts if it needs to clear its nose, nickers to express delight, and a neigh is sort of a "hey, I am here" call to other horses (or people). A horse neigh immediately followed by a nicker is possibly the best thing you can hear from a horse, for that is the horse saying "I'm so very happy to see you!"
But you'd never hear that on TV, because I guess the stock audio libraries they pull their sounds from don't have it.
Sigs are rather pointless, except if it's to showcase animation and images.