This guide is based on wikipedia's list of birds of Germany with accidentals removed to keep the guide under 500 entries.
The Brant or Brent Goose (Branta bernicla) is a species of goose of the genus Branta. The Black Brant is an American subspecies. The specific descriptor bernicla is from the same source as "barnacle" in Barnacle Goose, which looks similar but is not a close relation.
The Canada Goose (Branta canadensis) is a goose with a black head and neck, white patches on the face, and a brownish-gray body. Native to arctic and temperate regions of North America, it also occasionally migrates to northern Europe, and has been introduced to Britain, New Zealand, and other temperate regions.
The Barnacle Goose (Branta leucopsis) belongs to the genus Branta of black geese, which contains species with largely black plumage, distinguishing them from the grey Anser species. Despite its superficial similarity to the Brent Goose, genetic analysis has shown it is an eastern derivative of the Cackling Goose lineage.