I don't know if it has not been implemented so that people who are not added to your island can ride horses. But, only people who are added to your island should be able to ride any mobs. The reason being is that some players **couch** Anth___**cough** ride on the horses, and purposely kill them to get their saddles, and saddles do cost money, I am hoping that this feature can be removed so that only added players can ride horses. I have lost a couple of saddles because of that so it would be nice if you removed that feature. Thanks.