That is great of you to be thinking of your horse's needs! I would actually recommend a goat. Another horse or miniature will require the same care as your horse would. A goat can require lesser care (you can trim their hooves easily yourself), plus have the added benefit of keeping junk eaten down around your property. Goats and horses can bond just fine, if you introduce them carefully.
If you don't mind the upkeep of another equine, what about a pony? They probably cost less than true miniatures, unless you might be interested in showing minis at some point. Mules are very cool, too. :)
The benefit of a cow is it can eat your old hay you couldn't feed your horses. ;)
Ha--Brighteyes was typing at the same time I was! Ditto her comments on the mules/donkeys. Donkeys are awesome. Everyone should have a donkey.