Mares usually protect their foals and may resort to aggression in their defense, be it against other animals or humans. They should be given adequate time to bond away from other horses and people, and should not be reintroduced into groups that include individuals liable to be excessively nosy, pushy or aggressive.
In mares, ovarian granulose cell tumors also produce excessive testosterone and they can become aggressive too. Some mares will become aggressive during their seasons. If the mare is not to be bred, spaying her may be curative. For others progesterone or synthetic steroid hormones or some herbal remedies may help reduce the aggression and other unwanted behaviors
By Linda Aronson, DVM, MA