At the end of medieval times, writers talked about a semi-mythical ruler in Africa-“Prester John”. Early explorers placed him in Timbuktu or in other North African cities…but as far as I know, there was never any such person.
Anyway, who started the myth? And, what was the factul basis for the myth?
Wikipedia: Prester John
Really, shouldn’t you at least read Wiki on something before asking for fill in here? 
A better question would be, “What was the cultural purpose for the ‘remote, isolated pocket of Christendom’ stories during the Middle Ages?”
One could - and should - ask the same questions about the Jesus myth.
There was a Christian Kingdom in Ethiopia, which was south of the Islamic realms. And there were the Mongols who were attacking the Muslims from the east. The Prester John myth apparently linked these two facts and started the story that there was a powerful Christian kingdom on the other side of the Islamic countries.
Ethiopia is still Christian but it is not a Christianity that Europeans of that time would recognise. So even when they found it, they still kept on looking for their fantasy that had to be a Great White King.
MODERATOR NOTE
If “one” were a mind to, he could always post the question in another thread.
samclem Moderator, General Questions
Prince Valiant has encountered Prester John a couple of times in the course of his adventures.
Since that Wikipedia article relies heavily on it, I should provide a link to The Realm Of Prester John, at the time the most thorough and best researched history of the legend. A marvelous book. Yes, that is the same Robert Silverberg who is the sf writer. A master of everything he touches. And as elegant a gent as exists in the field. Made himself rich at a penny a word. Incredible.
Ralph. Get off the Internet for a while and read this book.
There was also a Nestorian (schismatic Christian) kingdom somewhere in Central Asia prior to the Mongol irruptions. I’m not at all clear on the details regarding it (perhaps Tamurlane might be of help?), but it’s my understanding that it was a constituent element of the sources for the Presterr John legend – though itself overrun by th time the PJ legend “gained legs”.
Europeans recognized Ethiopa as Christian. Medieval Christianity wasn’t racially exclusionary, and in terms of rituals and practice, Europeans were certainly familiar with Eastern Orthodoxy, and the Ethiopian Church isn’t sufficiently different from Eastern Orthodoxy that if you saw the one, you wouldn’t recognize that it was related to the other. (And, of course, even if you put Prester John in Central Asia instead of Ethiopia, he’s not white either way.)
I think the kingdom of Prester John filled a psychological need for European Christians. Europe was seen as under threat by the Muslims, and there was the fear would overrun Europe. Prester John reassured people that, “Hey, here’s an empire that’s rich and strong, and even if the worst happens, Christianity will survive, and eventually Prester John will beat the Muslims and rescue us.”
There is an excellent book by Umberto Eco called Serendipities which has an essay “The Force of Falsity” which covers the letter of Prester John, well worth picking up and reading. Eco also deals with the issue in fictional terms in Bardolino.