I know this question is very broad, and I apologize for that. I'm going to try to specify a bit more here.
In modern days, I see fantasy maps all the time. In books, games, movies even, and more set in fantasy worlds, there will often be a map made along with it. I've even met people who just make maps with nothing to put them to out of the joy of the act.
Since I play D&D and other such roleplaying games with varying settings, I often am looking for maps that resemble cartography anywhere from the 10th to the 18th century. But I've always looked either for modern fantasy maps resembling those periods or historical maps from those periods. I'd never seen a fantasy map from before the 20th century.
So I suppose what I'm asking is if any of you who have studied historical art or literature or fiction or cartography from almost any time period know of the history of making maps of places that don't exist within your studied field. I know this is a very broad question with a very broad period range, but I'm not certain how to make it any more concise. I have been told that many map makers would put fictional locations or landmarks in maps of real areas so they would know if their work was stolen, but what about maps of purely fictional places? And if it is rather recent, is there any known record of around when the art of fantasy map making became popular?
I don't post here very often, so I'm sorry if this question is poorly worded. I will try to clarify my question as best I can. If you have any questions or comments or critiques about my question, please let me know, and I will try to address it.
Thank you very much for your time, and I look forward to seeing any responses I may get.
Fantasy map-making in the sense of fantasy fiction is something of a contemporary invention, but only because what we consider fantasy fiction only became defined as its own genre in the late 19th/early 20th century. However, people have been playing with imaginary cartography for centuries - Athanasius Kircher had a map of Atlantis in his Mundus Subterraneus (1644) for example. In this sense of fantastic elements in what was supposed to be an otherwise mundane and accurate map of a real part of the world, it was prolific throughout history.
A really good resource for this kind of thing is J. B. Post's An Atlas of Fantasy, which traces in pictoral form the mapping of various fantasy worlds, from a map of Eden in Wo Lag das Paradies (c. 776) to The Chronicles of Thomas Covenant the Unbeliever (1978). Post also has a brief essay in the front of the book "On Charting Unreal Realms" which traces the difference between "geographical speculation and literary fabrication." For a more academic treatment (minus the pictures), you might check out Mapping the Imaginary—Maps in Fantasy Role-Playing Games - although it's very light on the history.
The popularity of the creation of maps intended solely for fictional "sub-creations" (to use Tolkien's term) is largely due to the phenomenal success of The Hobbit (1937) and The Lord of the Rings (1954-1955), which led to a huge boom in fantasy publishing in the 1960 and 70s. Tolkien was far from the first or only author at the time to do so - it's notable that his friend C. S. Lewis also included detailed maps in The Chronicles of Narnia (1949-1954) - but the detailed fantasy maps in those works really popularized the concept.