I think that's a question for Mary. However, I believe horses CAN use the stalls on community lots (therefore, someone in an apartment could 'board' their horse at one of the large equestrian facilities. Mary is the one who's expanded her ranch onto two other lots but they are community lots. Although I believe her plan is for the daughter to take over one of the lots (she would revert it to being a residential lot).
What I've done is I have the Fitzwilliam Darcy ranch and then his son, Alexander, married and started his own ranch right next door. Eventually, I think one of the grandchildren will take over the lot across the street and with MAJOR renovations, will also start his own ranch. But that's a ways down the road.