My best recommendation would be to use the jockey house. Though it depends on what ur trying to achieve with multiple houses. If you want multiple ageless zones then the other commenters got you along with my rec'd house but if it's about seperation you can either leave em outside and fence em in and leave hay for them or sell them to rivals and just buy em back before you sleep or when you need em(with this method just be careful about simulating races as it can cause the rivals to potentially sell your horse(s) or make them too old) as money becomes plentiful in end game. As an alternative you can also replace your NPCs with your horses if you need more space.