The University of Sussex has done some great research on how horses communicate using facial expressions, just as humans do. In fact some of these expressions are similar between humans and horses. Others are different, such as the horse's use of its very mobile ears.
The research has been done by Professor Karen McComb and her students at the Mammal Vocal Communication and Cognition Research Group. Here is part of a Science Daily news article about some of their work:
"Horses are sensitive to the facial expressions and attention of other horses, including the direction of the eyes and ears. The findings are a reminder for us humans to look beyond our own limitations and recognize that other species may communicate in ways that we can't, the researchers say. After all, human ears aren't mobile."
https://www.sciencedaily.com/releases/2014/08/140804123009.htm
They even found that horses are able to distinguish between angry and happy human facial expressions. Here is an article about that work:
https://www.sciencedaily.com/releases/2016/02/160209221158.htm
The point for us here is that the horse's understanding of other horse's (or people's) expressions is instinctive, not learned. For that matter it is instinctive in people as well. There are a lot of different expressions, so this is a complex body of instinctive knowledge.
McComb's group has even done impressive anatomical work toward cataloging these expressions. See this article:
https://www.sciencedaily.com/releases/2015/08/150805144820.htm
We have certainly observed a lot of facial expressions in observing our horses, especially the ears, which can be quite active. Given this research we will pay more attention to these expressions and the behaviors they are involved with.
It would be very interesting to know, in some sense, how many expressions there are and what they mean. Again, the point is not that the horses know how to do these expressions, but that they know what they mean when other horses do them.
The expression itself may not involve knowledge, but the communication certainly does. This seems like a sizable body of instinctive knowledge. As with our other examples, imagine trying to build a robotic horse that responds correctly to the facial expressions of real horses. No mean feat that would be.
