Assuming that foals are produced by a simple random process centered at 50% males / 50% females, a herd of 150 adults should contain between 37.75% and 62.25% females according to this relationship, where n = 150 and p-bar = .5.
Converting to integers, the number of female adults should range between 57 and 93.
Given that twins are very rare for horses, a herd of 150 adults should not be able to produce 100 foals unless something has occurred that invalidates the assumption of 50% males and 50% females.
Your host sent an email to the Chincoteague Volunteer Fire Company this morning seeking the current size of the herd and the number of adult males and females.
Western Horse Watchers Association 