I suspect that any herd/pack animal necessarily has a large range of social interaction signals to facilitate communication, regardless of whether those are noticeable for most humans (and I've seen the amazing Monty in Martin Clunes Horses series). I'm not altogether convinced though that horses have the same level of behaviour/capabilities developed specifically through interaction with humans which dogs appear to have.
As always I'm not a person who studies this so I'm open to being proven wrong but my viewings/readings so far bring me to the conclusion above ; )
The interesting thing is that the horses appear to be able to understand Monty's version of their own language. Whereas dogs seem to of developed two languages, one that they use between themselves & one that they use with humans.
Given the dog's ability to recognise & memorise new objects, I wonder if anyone has tried to teach a dog to use a symbol board as they do with apes.