The Hercynian Forest was an ancient and dense forest that stretched eastward from the Rhine River across southern Germany and formed the northern boundary of that part of Europe known to writers of antiquity. The ancient sources are equivocal about how far east it extended. All agree that the Black Forest, which extended east from the Rhine valley, formed the western side of the Hercynian.
Across the Rhine to the west extended the Silva Carbonaria and the forest of the Ardennes. All these old-growth forests of antiquity represented the original post-glacial temperate broadleaf forest ecosystem of Europe.
Relict tracts of this once-unbroken forest exist under many local names: the Schwarzwald ("Black Forest"), Odenwald, Spessart Rhön, Thüringerwald (Thuringian Forest), Harz, Rauhe Alb, Steigerwald, Fichtelgebirge, Erzgebirge, Riesengebirge, the Bohemian Forest, and the forested Carpathians. The Mittelgebirge seem to more or less correspond to a stretch of the Hercynian mountains, with Old High German Fergunna referring to the Erzgebirge and Virgundia (cf. modern Virngrund forest) to a range between Ansbach and Ellwangen.
The name is cited dozens of times in several classical authors, but most of the references are non-definitive, as the author is assuming the reader would know where the forest is. The earliest is in Aristotle (Meteorologica), who refers to the Arkýnia (or Orkýnios) mountains of Europe, but tells us only that, remarkably in his point-of-view, rivers flow north from there.
During the time of Julius Caesar, this forest blocked the advance of the Roman legions into Germania. His few statements are the most definitive. In De Bello Gallico he says that the forest stretches along the Danube from the territory of the Helvetii (present-day Switzerland) to Dacia (present-day Romania). Its implied northern dimension is nine days' march. Its eastern dimension is indefinitely more than sixty days' march. The concept fascinated and perhaps frightened him a little, even if the old wives' tales of unicorns and of elk without joints, which leaned against trees to sleep in the endless forests of Germania, were later interpolations in his Commentaries. Very likely, today's concept of an endless Black Forest descends in large part from Caesar. His name for the forest is the one most used: Hercynia Silva.
Pliny the Elder, in Natural History, places the eastern regions of the Hercynium jugum, the "Hercynian mountain chain", in Pannonia (present-day Hungary) and Dacia. He also gives us some dramaticised insight into its composition, in which the close proximity of the forest trees causes competitive struggle among them (inter se rixantes). He mentions its gigantic oaks. But even he— if the passage in question is not an interpolated marginal gloss— is subject to the mythological aura exuding from the gloomy forest. He makes mention of unusual birds, which have feathers that "shine like fires at night". Medieval bestiaries named these birds the Ercinee. The impenetrable nature of the Hercynian Silva hindered the last concerted Roman foray into the forest, by Drusus, in 12-9 BCE: Florus asserts that Drusus invisum atque inaccessum in id tempus Hercynium saltum patefecit. 
The isolated modern remnants of the Hercynian Forest identify its flora as a mixed one; Oscar Drude identified its Baltic elements associated with North Alpine flora, and North Atlantic species with circumpolar representatives. Similarly, Edward Gibbon noted the presence of reindeer— pseudo-Caesar's bos cervi figura— and elk— pseudo-Caesar's alces— in the forest. The wild bull which the Romans called the urus was present also, and the European bison and the now-extinct aurochs, Bos primigenius. 
In the Roman sources, the Hercynian Forest was clearly part of ancient Germania. We do find an indication that this circumstance was fairly recent; that is, Posidonius states that the Boii, who were Celtic, were once there (as well as in Bohemia).
In fact "Hercynian" has a Proto-Celtic derivation, from perkuniā, later erkunia. Julius Pokorny lists Hercynian as being derived from *perkʷu- "oak" (compare quercus). He further identifies the name as Celtic. Proto-Celtic regularly loses initial *p preceding a vowel, hence Hercynia (the H- being prothetic in Latin, the Latin y signifying a borrowing from Greek). The corresponding Germanic forms have an f- by Grimm's Law: Old English firgen "mountain", Gothic faírguni "mountain range". The assimilated *kwerkwu- would be regular in Italo-Celtic, and Pokorny connects the Celtiberian ethnonym Querquerni, found in Galicia.
It is possible that the name of the Harz Mountains in Germany is derived from Hercynian, as Harz is a Middle High German word meaning "mountain forest." The name of Pforzheim (Porta Hercyniae) in southwest Germany and the tiny village of Hercingen are also derived from Hercynian.
Hercyne was the Classical name (modern Libadia) of a small rapid stream in Boeotia that issued from two springs near Lebadea, modern Livadeia, and emptied into the Copaic lake. It had no geographical connection to the Hercynian Forest, so, logically, it must have been a parallel derivation from similar etymology.
The German journal Hercynia, published by the Universities and Landesbibliothek of Sachsen-Anhalt, covers ecology and environmental biology.