Animals have even been trained to deliver mail. Pigeons and doves have been used to deliver mail for at least a few hundred years. Ravens have also been used to carry mail for a few hundred years. Horses have long carried mailmen on horseback or in carriages. A very famous moment in history when horses carried mailmen was in the mid to late 1800's called the Pony Express. Today, sea lions are used to carry underwater mail from divers and oceanographers.