New research published in Biology Letters suggests that, similar to dogs, horses may understand our facial expressions. In a quote published in Discovery News, study author Amy Smith (University of Sussex) said, "It's possible that horses developed this ability during their 6,000-year co-evolution with humans, or indeed that individual horses learn it during their lifetimes."
To examine this question, the research team showed images of men with negative or positive facial expressions to 28 horses:
AV Smith, L Proops, K Grounds, J Wathan, K McComb. Functionally relevant responses to human facial expressions of emotion in the domestic horse (Equus caballus). Biology Letters. 12(2): 2016. DOI: 10.1098/rsbl.2015.0907