Unlike crows, horses do relatively little vocal communication. Of course they do sometimes whinny, nicker, snort, etc., but this is not the mainstay of their communication. As herd animals they are naturally in close contact, so they rely on what I call posture and gesture for communication.
For example, they make faces at each other, a lot of different faces, each with its own meaning. They frequently push one another, but without touching, by simply approaching in a certain way. They can threaten to kick by reversing, or mock strike with a front foot, or threaten to bite, etc. They can also groom one another, or play the face biting game, etc. Sometimes simply being close by is a form of friendship.
The point is that there is a huge amount of communication going on among the horses. This is easy to miss because humans tend to equate communication with human communication, which is dominated by verbal exchanges, just as with the crows. So-called body language is recognized to occur, but its role in human communication is relatively minor. With horses it is the primary means of communication, and there is a lot of it.