Horses are stoic animals, especially when it comes to dental pain. They often show us only minor signs of discomfort, which can quite commonly show up as behavioral problems.
For example, one horse owner told me she purchased a young horse who settled in well but had the strangest behavior while eating grain. After each bite, the filly stretched and twisted her head while chewing, spilling feed everywhere. The owner scheduled a dental exam with her veterinarian, and it revealed multiple tooth issues causing the horse pain. A few days after an exam and treatment, the filly was eating her feed normally.