By providence I just happened to read these excerpts one day apart:
"I believe one of the reasons we're all [referring to all living creatures] able to survive together is that we have a basic understanding of one another. For instance, any animal that has been fearful will likely recognize fear in another animal, even if that animal is a different species. Any animal that has been angry will recognize anger in another animal. By the same token, I believe that any animal that has ever wanted to get along with another animal will recognize when another animal is trying to get along with it.
I might take that another step further and say that if another animal does not pose a threat, then the first animal might be able to act more like itself, showing its true personality, rather than acting in a defensive or aggressive manner. Basically, the animal can be who he is.
It seems to me that when that happens, a door opens for communication between animals of different species and, in some cases, even between animals that are dire enemies... It's not uncommon in the animal world for different species to interact and even teach things to each other - particularly when it may benefit both species on a long-term basis."
-Life Lessons From a Ranch Horse by Mark Rashid
"Here, we have a highly strung, nervous and sensitive animal [the horse], who has to be trained in a manner whereof he cannot know the purpose, and who has to be spoken to in a language which is, to begin with, completely unintelligible to him. Moreover it happens all too often that the particular human being who is to do the training, is himself a stranger in the use of the horse's language.
For the horse, most definitely, has a language of his own; all animals of the higher order possess a language, or rather a method of communication. It is simple and is based on the expression of feelings and intentions by attitude and behavior. Dogs possess this aptitude in a marked degree, which makes it comparatively easy to understand them, for us as well as animals of other species.
It is perfectly certain that animals of different species can understand each other quite well and can and do communicate amongst each other satisfactorily, provided only that fear has no part in the proceedings; that there exists, in other words, a degree of confidence. It is most important to realize these facts. They imply that we shall not be understood by the horse unless we possess ourselves, or acquire, the aptitude of talking to him by attitude and behavior; and even then we shall never succeed fully unless we do so in that calm and quietly determined manner whereby confidence is inspired."
-Dressage: A Study of the Finer Points of Riding by Henry Wynmalen