I had a lot of trouble placing the equestrian centre in Barnacle Bay. I had to find a lot flat enough that the entrance would have a flat path for quite a way - otherwise, horses cannot enter. I played around with two custom-placed lots that were too hilly, before I moved a park that wasn't placed by me, and put the equestrian centre there instead. So my suggestion would be to find a lot with a big open flat space, move whatever's on it somewhere else, and place the equestrian centre there.