The Hercynian Forest was an ancient and dense forest that stretched eastward from the Rhine River. The ancient sources are equivocal about how far east. All agree that the Black Forest formed the western side of the Hercynian.
Ancient References
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 Arkunia ore (Hercynian mountains) of Europe, but tells us only that 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 (Book 6, Chapters 24 and 25), He says that the forest stretches along the Danube from 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 60 days march. The concept fascinated and perhaps frightened him a little. He entertains old wives' tales, such as unicorns in the endless forests of Germania. The Romans may have drawn that conclusion from the horns of narwhales used by the Germans. Very likely, today's concept of an endless Black Forest (which is far from reality) descends from Caesar. His name is the one most used: Hercynia Silva.
Pliny the Elder in Natural History (the paradigm for all subsequent natural histories), which also includes geography, places the eastern regions of the Hercynium jugum in Pannonia (present-day Hungary) and Dacia (Book 4 Chapter 25). He also gives us some insight into its composition. It contains gigantic oaks, he says (Book 16 Chapter 2). But even he 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. Edward Gibbon noted the presence of reindeer and elk in the Hercynian.
The wild bull was another animal in the Hercynian Forest: the aurochs, bos primigenius, which they called the urus and ureox. Although the aurochs became extinct since Roman times, tt has, however, been genetically reconstituted in the forests of northeast Poland and Belarus. These forests are not believed to be have been continuous with the Hercynian, as the flatlands of Poland intervene. With animals such as the aurochs and brown bear and Germanic tribes such as the Suevi in the Hercynian, it is no wonder that the forest generated such awe in the superstitious Roman soldiers.
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 Celtic derivation.
Etymology
Julius Pokorny lists Hercynian as being derived from *perkwu-, "oak", where -kwu- represents Pokorny's labio-velar. He further identifies the name as Celtic. Regarding the H-, Celtic forms usually lose an initial Indoeuropean *p-, and the corresponding Germanic forms have an f- by Grimm's Law: English fir, Gothic fairguni ("mountain"). Pokorny therefore hypothesizes a development from the assimilated Indo-European form, *kwerkwu- (Latin quercus), as in the Celtic people Querquerni in Hispania. Germanic speakers would have made an h- of the *kw-. Hercynia would thus be a Germanicised Celtic word, testifying to the former dominance of the Celts along the Danube.
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."