In Irish mythology, Breg was the wife of the Dagda.

The Breg is also the longer of the two German rivers that form the Danube river. Its spring is situated at 1078 m above sea level in the Black Forest, near Furtwangen, 100 m away from the invisible line that forms the European Watershed . After 49 km, it joins the Brigach river in the city of Donaueschingen and is henceforth called Danube.