AWithout further information, I can offer explanations for a couple of likely scenarios. If the mare is just buddying up with another mare along a common fence line, it could simply be social behavior. Some pairs just have an affinity for one another; they would buddy up if they were allowed direct contact, and still do despite the fence. But I am guessing that there is more to it than just standing near one another.

A common complaint is a mare that shows stallion-like interest in other mares, and might also behave with males as if she were another male. This would include prancing and vocalizing with an arched neck like a stallion, sniffing expelled urine and feces of other horses, posturing to urinate or defecating over the excrement of other horses, sniffing a mare in the characteristic teasing pattern of a stallion, performing the flehmen response, and even herding or mounting other mares.

In the case of mares, any stallion-like behavior is abnormal and reflects exposure to male hormones or very high levels of estrogens (which can induce male-type behavior in the mare). There are two common culprits. One is steroids given to enhance performance. Race fillies are commonly given anabolic steroids or testosterone to boost their athletic performance and increase muscle mass. These drugs can make a mare’s behavior almost indistinguishable from that of a stallion in some cases, and the stallion-like behavior can linger for as much as six months to a year after the medications are no longer given. If you have owned this mare for a year or more and she has had no steroids administered, then you can probably rule that out as a cause