Miniature horses have a tendency to over eat. This is really and I mean really bad for their feet. A good idea would be to only let your horse out to graze for an hour or two a day or to ration his feed. Muzzles for any size horse are very hard to come by.
A grazing muzzle, shaped like a bucket, fits over the nose and mouth with a small hole in the bottom, restricting the amount of grass the horse can eat.
Look for a grazing/grass muzzle, sized and designed for miniature horses that attaches to the halter in a basket weave pattern so that water can still be consumed.