Prester John was a legendary Christian priest and king who is supposed to have lived in the 1100s. Many travellers and explorers of the Middle Ages, including Marco Polo, claimed that he ruled a vast kingdom in central Asia. The legend arose during the period of the Crusades (late 11th to the 13th century), but historians have long placed Prester John within the category of myth.
However, in an interesting post on Time to Eat the Dogs, Michael Robinson writes: "The point here is not to say that Prester John was real, but to say that in characterizing him as a mythic figure, historians have tended to discount his serious influence on European exploration and discovery. This is a central argument of historian Michael Brooks in his excellent thesis, Prester John: A Reexamination and Compendium of the Mythical Figure Who Helped Spark European Expansion".