My 1st horse spent her time in the field with the cows for a few years before we bought another horse. Even with two, she enjoyed being with the cows. She was the boss there and even pushed the herd bull around.
The might function alone, but they will be better off with other herd animals around. Even if it's in an adjacent field. She almost always see some of the caws even if they happen to be in separate feilds. The 2nd horse was just another playmate to her.