What are the sounds that a horse makes and why do they make those sounds?

A horse can whinny, scream, nicker, and even low growl. When a horse nickers it can mean that it likes something, or when its feeding time, they nicker to there feeders. When it screams it can mean a warning, an urgent call, or (GET AWAY!!!). When a horse whinnies, it may be trying to find something, answering another horse, or letting something (or someone) know that it is there.

The information below is referenced from this site:
http://www.sportpolo.com/polo_pony/Sounds.htm

The Neigh or Whinny

A neigh sounds like a squeal followed by a nicker and is very loud.

The term whinny is also used to refer to a neigh.

When a horse neighs (or whinnies) with his head high, he is looking for other horses or people.

The Nicker

The nicker sound is produced when a horse creates a vibrating sound with his mouth closed using the vocal cords.

When a horse nickers quietly, and moves toward a horse or person, he is saying "hello".

When a horse nickers intensely and shakes his head, he wants to mate.

When a nicker is much softer and quieter than a "hello" nicker, it's usually made by a mare to her foal.

The Blow

The blow sound is produced when a horse exhales through his nose with his mouth shut.

A horse blows when it's curious, or when greeting another horse.

When horses meet nose to nose and blow on each other, they are greeting each other.

• If the horses continue blowing gently followed by other actions such as nuzzling, they are friends.
• If one or both of the horses nip at the other, stomp their front feet, strike out, or squeal, it means they are enemies.

When a horse blows at something he usually curious.

• If the horse relaxes all is well.
• If the horse tenses up or shies away, it senses danger.

The Snort

The snort sound is produced when a horse exhales through his nose with his mouth shut and produces a vibrating sound in the nostrils. Very often the horse holds his head up while snorting.

When a horse snorts and stares at an object he is checking for danger.

The Squeal

A horse squeals with his mouth shut.

When a horse squeals it means no.

The Scream

A horse screams while fighting with another horse.