My old mare Promise had a silent nicker she used as a greeting - we called it "doing nostrils" - her nostrils would move but there was no sound. And Lily had a funny sound she made when getting to know another horse - she would sniff noses, and then give a short, very high-pitched "me" while turning her head away (in contempt of the obviously lesser being). And Norman the pony would, when excited, give a loud whinny always accompanied by explosive farting - I guess all that effort had to make sound from both ends!
But Dawn has more variety in her expressions. When she's bolting from the gate, or excitedly herding the other horses, or angry at a gelding for expressing interest when it's (not quite) the right time, she does a loud "eeeEEE!", usually accompanied by a head shake, a bite or a kicking motion. She'll do this in her stall when she's in heat and kicks the wall. When she's interested in another horse, she'll do a quieter, interrogative "eeEee" - it has a very come-hither sound to it. She does a bellow and stamp when she's intimidating another horse face to face.
She has a low, sexy nicker that she uses to greet human and horse friends, usually accompanied by sniffing and nuzzling - I think it's about the sound a mare would make to a foal. She oddly enough often makes this sound for our farrier - he finds it somewhat disconcerting. She has what we call her "frantic whinny" - this is used when she's calling to another horse from a distance or is worried about an absent horse.
And she grunts. She grunts "unh" - a short, fairly loud exhalation, sometimes when she bucks but also if she unexpectedly bumps her nose into something - she often is looking in a direction other than the one she's walking in and can bump her nose into objects like door frames or fences. It has a bit of a sound of protest about it.
And last week, she hummed. That's the only word I can think of that describes it - it was the same "eee" sound, but soft, low-pitched and elongated - just softly sing "e" in a quiet, low voice and that's the sound she made - she was using it to acknowledge Maisie as we walked by her paddock.
I wonder what she'll come up with next?