This is rather unusual but very easily treated and shouldn’t be left untreated as they will compensate through the neck and back when there is pain in the poll.
In 30 years of treating horses, I have only seen this a handful of times.
Horses tend to be irregular in their head carriage, tilt the head and be reluctant to take the bit when they have asymmetry of the facial muscles. They palpate sore over the poll and generally struggle with correct upper neck flexion.
The first picture shows asymmetry before treatment. The second picture has been taken after initial treatment and the third picture has been taken after the second treatment, two weeks later.
