I can tell you without a doubt you do not own an inherently grumpy horse. You own a horse who is upset - experiencing some level of physical and/or emotional distress.
Horses have very expressive faces. When we see pinned ears, tight lips, and drawn cheeks, there's most definitely something wrong. Horse's faces don't freeze in that position, like our mothers used to warn us.
If this is a brief change in your horse's face, instead of a usually relaxed and happy countenance, it's more likely to be a normal response to a short lived unpleasant experience, like another animal getting too close or a brief moment of discomfort or confusion.
If your horse's face looks tense for any length of time, or seems generally put off by life, it's a sign of chronic pain and/or stress. Pain (the most common cause), confusion, or some other form of upset can be seen on your horse's face.
Check It Out
The cause needs to be investigated, pronto. Your horse isn't likely to solve the problem, or problems, without help.
Also, don't be deceived into thinking your horse has a sour attitude, but there's nothing really wrong.
Is Your Horse Afraid to React?
Horses learn, because we teach them through pain and force to adopt feelings of helplessness...that they have no choice but to soldier on. We can condition a horse to suppress natural avoidance responses to things they find unpleasant.
That means they will allow us to damage them if they know that complaining leads to punishment. That can create the chronic "grump," a horse who isa suffering with a genuine problem, not a spontaneously random bad attitude toward life.
Learn more about understanding, listening, and helping your horse at Purejoyhorsemanship.com.