And by this I mean, was the "Medieval Ranger" archetype extant prior to the writings of J.R.R. Tolkien?
I'm not sure what is meant by the "Medieval Ranger" as an archetype. Such a thing is described on
this medieval re-enactment website. I wouldn't be surprised if it owes much of its existence to the D&D Ranger in the first place. Given that you're posting in an FRPG forum, I suppose you could be talking about the Ranger as a character class archetype as referenced in
this Wikipedia article. Once again, such a thing would most certainly derive from the D&D Ranger as the first of its type, all of this coming many years after the writings of Tolkien. Of course, this doesn't preclude the existence of figures that we might identify as falling into this archetype at some earlier time, but the labeling of any of these figures as "Rangers" is obviously quite modern and seems to be following the Professor's example.
As far as I understand, the medieval English ranger was a kind of game-warden that protected the king's forests from poachers and the like. I suppose, in some incarnations, Robin Hood could be seen as holding Sherwood Forest in the name of the true king, but I don't know if he was ever referred to as a ranger, falling much more into the archetype of the yeoman archer. And while the figure of the medieval ranger certainly stands behind Aragorn and Co., Tolkien's creation was so much more.
His use of the word
ranger is also quite nuanced. It not only has the historical meaning of a king's official forrester, but also describes the action of far wandering for which Aragorn is known. According to a comment on
this website, a third meaning derives from an archaic usage, synonymous with the word
rake, with the meaning of "an idle or dissolute person." Obviously, Tolkien's use of the word with reference to the character of Aragorn is an invention in itself, given the multi-faceted nature of the Dunedain Rangers as the shadowy representatives of an unknown king and guardians of civilization, inhabiting the wild places, seen by some as scruffy and homeless vagabonds. On this last meaning, compare the description of the Ranger found in the first sentence of the entry in the 5E PHB, p. 89, "Rough and wild looking". Can there be any doubt that D&D designers are still intentionally using Aragorn as a touchstone for the image of the class?
I believe that Tolkien coined this particular usage of the term
ranger, so the question, then, is whether we can find any evidence that Joe Fischer modeled
his Ranger on the Professor's work, or if the similarities are superficial, or even coincidental. Joe Fischer's
unofficial Ranger appeared in The Strategic Review #2, in the Summer of 1975, but it is nearly identical to the official version that showed up in the 1E PHB. Minimum ability requirements included a Constitution of 15, reminiscent of Aragorn's legendary toughness. The first real clue that the Ranger is meant to emulate Tolkien, however, is the second level class-title, "Strider." This type of cursory reference to Tolkien is found throughout D&D, 1974, so it isn't surprising. EGG, himself, later admitted to having included elements of Tolkien's work to induce fans of Tolkien to try his game. What might be surprising is that Gygax left the reference in when making the Ranger official, at a point when many of the references to Tolkien had been expunged, or at least obscured.
Gygax moved away from Tolkien, however slightly, when he replaced the Cleric spells that Fischer had given the Ranger at level 8, with Druid spells, possibly to put the Ranger more into the mould of a warrior aligned with the forces of nature, an idea that is not as present in Fischer's version. The Clerical spells had been included by Fischer, presumably, to mimic the healer's arts possessed by Aragorn, as was the ability, gained at the same level, to use magic items that heal or cure disease.
Then there are the restrictions on possessions and hirelings; Aragorn had no possessions to speak of and did not travel with a body of men-at-arms or servants. Also, there is the restriction on more than two Rangers operating together. Aragorn undergoes all of his formative adventures alone, and only in the events leading up to the climax of the novel is he joined by others of his order.
Then there is the superior tracking ability of the Ranger, which made its way virtually unchanged into the official version. This is textbook Aragorn in one of his greatest moments, i.e. tracking the Uruk-hai across the plains of the Eastfold, not to mention the tracking and capture of Sméagol.
Next we have the fighting advantage versus "giant class" humanoids. This emulates not only the many battles against orcs throughout the book, but also the climactic event of Aragorn's trajectory as an action hero, the single-handed slaying of a black troll at the Battle of the Black Gate, in which he nearly loses his life. The wounding of the troll's foot as it comes down on Aragorn's throat mirrors the wounding of the heel of Morgoth when he fought with the elven-king Fingolfin in giant form, so it's an important, symbolic moment in the book.
There is the ability to use all magic items that deal with telepathy, etc., gained at 9th level. Compare this with Aragorn's claim to the Palantir of Orthanc.
Finally, there are the followers. Among the possible results are a dwarf, an elf, and two hobbits, as if upon attaining 8th level, the Ranger will attract its own "Fellowship" to lead through the wilderness on an epic quest. The inclusion of a lawful werebear in the list of possible followers also seems to be a clue to the reader that this is a Tolkien-themed character class.
As I said above, much of this made its way into the official version relatively unchanged, and much of it remains, in some form or another, even in the 5E version. I hope Joe Fischer was paid well for his ideas, but I think it's pretty obvious where he drew his inspiration from.