From Helleiner, "Prester John's Letter: A Mediaeval Utopia":
“Let us proceed from the curious fact that a person called Presbyter Johannes is mentioned by Eusebius. In his History of the Christian Church this celebrated fourth-century author quotes from an early Christian writer, Papias, who names as on of his authorities “John the Elder”… Eusebius’ Church History was widely read in the Latin translation of Rufinus, which contains the words Iohannes presbyter… [I]n view of the fact that man seems to exercise his mythopoeic faculty by preference on familiar subjects the demonstration that a figure called Presbyter Johannes was known before this name came to be applied to a half-legendary ruler of the Indies may not be without significance” (53).
From Silverberg, The Realm Of Prester John:
“[I]t is important here to consider the existence of a persistent apocryphal tradition that the Apostle John did not die but was chosen by Jesus to wander in the world as an immortal until the Second Coming of Christ” (37).