A mule or courier is someone who personally smuggles contraband across a border (as opposed to sending by mail, etc.) for a smuggling organization. The organizers employ mules to reduce the risk of getting caught themselves. Methods of smuggling include hiding the goods in vehicles or carried items, attaching them to one's body, or using the body as a container.
In the case of transporting illegal drugs, the term drug mule applies. Other slang terms include Kinder Surprise and Easter Egg. Small-scale operations, in which one courier carries one piece or a very small quantity, are sometimes called the ant trade.