Whoa! You’re heading for some clouds on your property title!
Your property is defined by the deed description… do you have it in hand? If you allow your neighbor, with continuous use, to gain a portion of your land via a prescriptive easement, that could cause a cloud on your title for future land owners of your parcel. Just because they “straighten out the fence doesn’t mean the deed at the courthouse gets changed.
I can’t imagine any County Planning department allowing a subdivision of land without a survey and proper maps. The property split will have to have new proper deed descriptions written up, either by a subdivision map or metes and bounds description.
You don’t have to go to court to have this done, but you’ll likely need the assistance of a surveyor and perhaps an attorney. I’d suggest finding out the requirements of your County Planning and contacting a surveyor first.