Horses use 17 discrete facial movements in communication, compared to 27 for people, 16 for dogs, and 13 for chimpanzees. University of Sussex researchers determined this by studying the musculature under a horse's face and watched videos of horses of all ages and multiple breeds. This enabled the scientists to create a catalog of facial behavioral sequences named EquiFACS (Equine Facial Action Coding System.) From National Geographic:
Jennifer Wathan, the study’s lead author, says the similarities between horse movements and human ones are striking. They include raising inner eyebrows (“puppy-dog eyes”) to show fear, surprise, or sadness; pulling back lip corners (smiling) in greeting or submission; and opening eyes wide to indicate alarm...
Her team’s research, which is already helping veterinarians and trainers, could also connect facial expressions to emotional states. “We don’t know much about the emotional lives of animals,” she says. “What does a positive emotion look like? This tool could help us see it.”
"EquiFACS: The Equine Facial Action Coding System" (PLOS One)