I believe a house can only be grandfathered in IF there are already horses on the property. If there aren't and the laws change, I don't think there is anything you can do.
I believe that Appy is right here. My grandfather has a 5 acre parcel in town...He used to live on the edge of town, but town grew around him. City hall passed ordinances that stated there could be no livestock kept "within city limits" BUT, since he had hogs, cattle and a horse on his land, that land as long as it's kept in the family, has grandfather rights to it and can always be used for livestock. He has gotten rid of the hogs (no pig poo makes for happier neighbors) and the cattle (he's getting to old to work them) but has kept his horse.
If your grandmother didn't have livestock when the new laws were made, you may be out of luck. Sorry.
I will say that having the horses here at home is wonderful...but there are sacrafices. Like getting to go camping...or spending the night somewhere on a whim. The horses need fed 2x/day and checked on regularly. We always have to concider the horses when making plans. I'll tell you from experience, it's not always easy to find someone that you can rely on to do your chores while you are away.