“Until now the full capacity of horse facial expressions to convey a range of information has been largely overlooked.”
This lack of capacity was addressed by the most recent (2015) addition to the AnimalFACS system. (FACS stands for Facial Action Coding System)
EquiFACS which is
“a scientific observational tool for identifying and coding facial movements in domestic horse (Equus caballus).”
– has defined 17 defined Action Units (Aus) for horses – which, for comparison, is 10 less than humans and 4 less than cats, but 1 more than dogs)
Details of the system can be found in: Wathan J, Burrows AM, Waller BM, McComb K (2015) EquiFACS: The Equine Facial Action Coding System. PLOS ONE 10(9): e0137818.