The wording „bock“ is not related with the buck [zool.]
In fact, the former Hanseatic city „Einbeck“ (close to Hannover) is the cradle of the bock-beer. Long ago in the 13th century a strong, top-fermented beer was brewed there. Due to the high alcohol content and original gravity it was very popular. Also it was quite nonperishable and was even exported to Italy.
Due to its excellent quality even the dukes in Bavaria liked the beer of Einbeck, but the import became too costly after a while. Therefore they decided to build an own brewery in Munich. In 1614 they headhunted the master-brewer Elias Pichler of Einbeck to recruit him in Munich‘s „Hofbräuhaus“. From then onwards the „Ainpöckisch Bier“ (a bock-beer) was brewed the „einpöckscher way” of which later the wording “bock-beer”, respectively in the 20th century the general term “double beer” emerged.