According to an article in Tuesday’s New York Times:
It was 1941, shortly before the invasion of the Soviet Union, and an anonymous source tipped off the Nazi authorities: A businessman named Tor Borg, of Tampere, Finland — a country that was friendly to the Nazis but not occupied by the Reich — had a black-and-white spotted dog that he taught to mock Hitler. The German vice consul in Helsinki, Willy Erkelenz, wrote that “a witness, who does not want to be named, said he saw and heard how Borg’s dog reacted to the command ‘Hitler’ by raising its paw.”
Read the full article here.