Horses should have company.
Herd animals, prey animals for that matter need to be in company. Horses groom , play and use each other as security-especially at night , and in a natural environment /wild they form pair bonds which last forever if allowed. Keeping them on there own doesnt allow for them to act out natural behaviours.....as far as interaction with other animals go.
I have read a study on a horse that lived on his own for a very long time, along with many other problems one related to being on his own was he often took any moment he was around people as a time to doze/ sleep as on his own he had to be on the look out so couldnt fully relax as he had no one to look out for ' danger ' for him while he rested.
True ....some horses can cope with it, but I bet if they had a say in how they lived they would choose company over not having any.
Even if you can't afford 2 horses at present, you could put up a paddock mate wanted add and see if you can get a companion that way, even for short term untill you get other horses or animals. The you will just be looking after your horse but he will still ahve company.
Honetly I hate seeing horses on there own. But really the choice is yours