There is no rule that says horses cannot wear a halter in the pasture when being turned out. However, there will be certain risks of injury. The horse can get caught in the halter and therefore it is deemed safer to let the horse walk in the pasture without it.
If you still want your horse to wear a halter in the pasture, it is a good idea to choose a leather halter that breaks in the event of an accident. Equestrian Stockholm offers leather halters of the highest quality in stylish designs.