If you want more than 1 horse, do NOT visit the stable lv 5, craft one then go immediately to either the workshop or the magic forge and pay 999 gems per horse. If you do not have the gems right now or if you want to keep the option to buy more horses open indefinitely either STORE both lvl 5 stables (I know you sold one already) or sell them and rebuild to level 4, in order to be able to continue collecting milk, hide, wool and bacon without finishing the quest and closing the option to buy more horses. If you already bought however many horses you want and you know you don't want any more, feel free to visit the stable lvl 5 and get the quest out of your quest book.
Hope that Made it more clear :-D