The German Shepherd was a breed of domesticated dog native to Earth. They were used as guard dogs by the United States military during the 20th and 21st centuries.
After being accidentally sent back in time from 2372 to July 1947, Constable Odo disguised himself as a German Shepherd while Quark, Rom and Nog were taken prisoner by the US Army Air Corps and held in the Roswell Army Air Field. In the form of the dog, he was able to eavesdrop on conversations and sneak around the base with impunity. He determined that Quark's Treasure was being stored in Hangar 18 of the facility. (DS9 episode: "Little Green Men")
In 1974, German Shepherds were used as guard dogs at Brandenburg Gate in East Berlin. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 1)
In 1986, a German Shepherd was used as a guard dog aboard the USS Enterprise. (TOS movie: Star Trek IV: The Voyage Home)
In 2371, an illusory German Shepherd attempted to keep the crew of the USS Voyager from exploring what appeared to a barn aboard the Caretaker's array. (VOY episode: "Caretaker")