I believe you must know where the border is before you go in and movey our horse into that area. I have not tried is as I have enough maids that can drop my trash off in the storage following their CD timer. If you have storage maids that is what you can do as well. But for as far as I understand it you need to bring your horse with you into the Marni Realm (within its borders, basically). If that doesn't work then I'm sorry, that's all I've got.