Once in a while a herd stallion will venture out to a small group of bachelors to 'show them who is boss'. It seems a necessary action to maintain his upper ranking over the un-attached males.

In this image, the white stallion (Ice) has chosen a big black stallion to harass. Even though the black male is as old as the lighter horse, he has yet to win a mare, and therefore a threat to the dominant male.