Publius Cornelius Tacitus in his De Origine et situ Germanorum tells us that 1st century Germanic tribes worshipped Hercules:
They say that Hercules, too, once visited them; and when going into battle, they sing of him first of all heroes. They have also those songs of theirs, by the recital of which (“baritus,” they call it), they rouse their courage, while from the note they augur the result of the approaching conflict. For, as their line shouts, they inspire or feel alarm. It is not so much an articulate sound, as a general cry of valour. They aim chiefly at a harsh note and a confused roar, putting their shields to their mouth, so that, by reverberation, it may swell into a fuller and deeper sound. Ulysses, too, is believed by some, in his long legendary wanderings, to have found his way into this ocean, and, having visited German soil, to have founded and named the town of Asciburgium, which stands on the bank of the Rhine, and is to this day inhabited. They even say that an altar dedicated to Ulysses, with the addition of the name of his father, Laertes, was formerly discovered on this same spot, and that certain monuments and tombs, with Greek inscriptions, still exist on the borders of Germany and Rhætia. These statements I have no intention of sustaining by proofs, or of refuting; every one may believe or disbelieve them as he feels inclined.
Are there other sources that discuss a Hercules cult in the general area? Or is this an instance of interpretatio graeca? And if so, which Germanic deity or hero is Tacitus substituting with Hercules?