according to Mandeville, a lineal descendant of Ogier the Dane. This Ogier penetrated into the north of India, with fifteen barons of his own country, among whom he divided the land. John was made sovereign of Tenedue, and was called Prester because he converted the natives. Another tradition says he had seventy kings for his vassals, and was seen by his subjects only three times in a year. In Much Ado about Nothing, Benedick says:
“I will fetch you a tooth-picker from the farthest inch of Asia; bring you the length of Prester John's foot: fetch you a hair off the great Cham's beard. rather than hold three words' conference with this harpy.” - Act ii. 1.
Prester John (in Orlando Furioso, bk. xvii.), called by his subjects Senapus, King of Ethiopia. He was blind. Though the richest monarch of the world, he pined “in plenty's lap with endless famine,” for whenever his table was spread hell-born harpies flew away with the food. This was in punishment of his great pride and impiety in wishing to add Paradise to his dominion. The plague was to cease “when a stranger came to his kingdom on a winged horse.” Astolpho came on his flying griffin, and with his magic horn chased the harpies into Cocytus. The king sent 100,000 Nubians to the aid of Charlemagne; they were provided with horse by Astolpho, who threw stones into the air, which became steeds fully equipped (bk. xviii.) and were transported to France by Astolpho, who filled his hands with leaves, which he cast into the sea, and they instantly became ships (bk. xix.). When Agramant was dead, the Nubians were sent back to their country, and the ships turned to leaves and the horses to stones again.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894