well its always good for an animal to have its own kind. Everything my fiance and I get are in multiples. We have 2 ferrets, 2 horses, 5 fiddler crabs. That way if we can't devote enough time to them, they'll have company and be able to use all their energy (or most of it) playing with each other.
A mini sounds like a good idea, or even if you get a companion horse that's a rescue. I also heard goats are good at keeping horses company, but I havent tried it so I wouldnt know.