https://www.heraldry-wiki.com/wiki/Giengen_an_der_Brenz
Giengen was a possession of the Staufen family and received city rights in the middle of the 13th century. Already the oldest seals, known since 1293, show the unicorn as the symbol for the city. Its meaning is not known. The arms of the city have always been the unicorn. The seals, however, showed the imperial eagle. Giengen was an imperial city and thus had the right to use the eagle. To distinguish the seal from seals of other cities, the letters G and I, later only G were added. In the 17th century both the arms with the unicorn and arms with the imperial eagle were used together.
When the city became part of Württemberg the imperial eagle was no longer used.