When I had my filly, Miss Melody, I wanted to think she was happy by herself but it became apparent that she was not. I had a hard time finding her a match but finally found a match in a steer and a goat. She ran with them for about 6 months before she went to live with her previous owner again. She did seem happy and she even played with them and was queen of her herd
The bad thing about having a young horse by themself is that they don't learn how to behave from the older, mature horses. I will never again have a horse by themself unless it's an older horse that prefers it that way. Babies need interaction and I learned that the hard way.
I have been doing a lot of thinking and am going to try and find 2 very small minis (29 or under) so one won't be alone. It is easier for me, and the wear and tear on the property.