OH MY GOD, please please please, NEVER NEVER leave a healthy horse on it's own in any situation. They MUST have a companion with them. It's tantamount to cruelty to put them on their own.
Sorry for all the capitals but any horsey person will tell you how wrong that would be.
Horses need to be with other horses. It is cruel to have them on their own. If you dont know this then you know nothing about horses and should not have one.
It's not so bad if the horse in question is ridden out two or three times day and kept in a paddock for short spells in between, but they enjoy the company of others - wouldn't you?! x