When adding a world save to a Realm that has more than one resource and or behavior pack it will automatically change a Horse back to its default Model. For Example: I have the Pets world bought from the marketplace that makes a Horse a German Shepard but I have also added a custom resource and behavior pack to this world save to convert spiders and cave spiders into go-karts this works fine when loaded in regular Minecraft but once its added to a Realm it works with the exception of changing the German Shepard back into a Horse. I have also noticed that if you are using multiple resource or behavior packs on a realm it will also change your texture packs back to default too.