A horse that's kept alone will welcome company, be it a human, dog, cat, goat. If a horse loves a person then it won't immediately run off or attempt to when another horse shows up? If the horse loves a human then it will be always obedient, never put the rider at risk with a buck or spook.
I think every horse has their own unique personalities and will display affection and emotion in their own way. My beloved mare T was very stoic and was the head mare in a group of about twenty horses. When I would go out to get her she would come to me but run the other horses off from being too close to me. At the same time she would follow my directions and certainly always minded her manners around me. Did she love me? I'd like to think she had affection for me. While she was opinionated and let me know, she would follow my wishes after expressing her own. Yes, she saw me as the leader, or did she see me as a worthy partner? I'd love to acquire that relationship with Walka or Misty, but may never because of their own unique personalities.
When T passed I personally watched Walka grieve for her. It was his final acceptance of her not physically being there any longer that helped me get a handle on my own grief. He had never known a moment without his mother except for the weekends she was away with me on overnight trailrides. When she passed he knew almost instantly and began screeching for her. Very traumatic, very emotional. When he called the last time for her the next day (he called for her throughout the night) after checking every possible place she could be, he came to me and put that big head on my chest to be rubbed and never uttered another sound. Simply amazing to me.
So I think each horse feels emotions but like people, there are many different personalities and they therefore demonstrate their emotions differently. My girl T kept alot inside, Walka let's you know exactly what's going on with him. Misty is still showing me her personality.