Horses are quite expressive, but their body language is alien. Horse facial expressions exist, but involve very quick eye movements, something discovered only about twenty years ago. Most of the bigger horse expressions are in ears, lips, tail, and carriage.

If horses don't get feedback from their humans indicating some understanding on the human side, they often stop trying to communicate. To learn how horses communicate, watch them in an open field for long periods. There are frequent subtle dominance displays. Not kicking, or even ears pinned back; often it's just slightly narrowed eyes and an eye-flick, and one horse will yield space to another. If a horse does that to you, putting hands on hips and giving the horse a hard stare says that you got the message and aren't backing down. Most horses get this. It's a compromise between horse body language and human body language; a pidgin, if you will. Horses recognize eye contact. You're much less likely to be bitten, even by a hostile horse, if you're looking at the horse's eye and they're looking at you.

It's quite possible to "speak horse" that way. "Talking with Horses", by Henry Blake, is useful.

Whether or not a horse understands human to human expressions depends on how much experience they have with humans. It's a second language for horses, after all. Horses that live in herd situations deal mostly with other horses, and may not be that attuned to humans. Ones in stall situations tend to be more people-oriented, by necessity. The parent article was about horses that deal with lots of different people, so they're likely to have figured out human expressions.

Probably not a coincidence that they're also one of the relatively few domesticated animals (https://en.wikipedia.org/wiki/List_of_domesticated_animals).

It's acting more like a dog than a cow. And there are many other videos of horses playing and doing tricks like dogs can. My opinion of horses greatly improved. Thinking about it, I shouldn't have found it surprising. Horses are herd/pack animals like dogs, so they naturally have social skills, and despite being herbivores they are still mammals, so their body language is at least somewhat similar to humans. And also like dogs, horses have been domesticated to work closely with humans. Most other domesticated animals are either solitary, or not required to work directly with humans. I think any domesticated mammal with both those properties will likely read human emotions. Even though the dog has the advantage of being a carnivore so it is more similar to humans, and being required to work more independently than the horse, putting more selection pressure on its communication skills, the horse still far exceeds popular animals like the cat in its ability to communicate with humans. If the dog is "man's best friend", I think the horse can be "man's second best friend".

The hot-blooded breeds (Arabs and Thoroughbreds) seem to be smarter, but that may be because they're more active and more likely to initiate activity.

They're also frequently much more emphatic than people I mostly work with as a programmer. ;-)

No, really: I own two cats now, and I grew up with dogs, cats, horses and goats around and it led me to believe that animals generally do dream, do have emotions and are generally able to understand or feel emotions of others. They also do think, they do try to predict the future, and they do learn on their own, sometimes frighteningly quickly.

But it's convenient to believe otherwise and even people who should know better tend to forget about it during the meals. Just try to imagine how eating a burger would feel with a thought like: "ok, now I'm eating something that was smarter and more emotional than my 2yo niece".

Behavioural studies in animals are a fascinating field. I can't back it up with citations right now, but I'm pretty sure horses and dogs are not the only animals to read human facial expressions. Chimps, cats, parrots - you name it - if it's been in close contact with humans for any length of time, I wouldn't be surprised if it learnt how to read our emotions.

Indeed. One wonders if the person holding the horse and the person presenting the face photos could see if the photo was of an angry or other face.

    Experimenters were blind to the stimulus
    in 82% of trials and analyses showed no 
    difference in subject responses according
    to this factor (see the electronic supplementary
    material tables SII and SIII).

Domesticated horses are generally trained through fear; it is not surprising they react to angry faces. So it might also be interesting to conduct this same test on horses trained only with positive reinforcement techniques.