German Shepherds were originally called German Herding Dogs, but the name was too vague and broad, so they switched it to the German Shepherd Dog. The breed was originally bred for it's working ability, not it's looks. therefor several german herding dogs were used, and there wasn't a really set look to the bread. broad look, broad name. When a reliable physical standard started to form, a set name formed.
There was no specific reason DOG was left in the name. German shepherds were bred to be the perfect working dog, perfect herding dog, so they became the german shepherd dog. And no, there are no other "german shepherds" out there. there are other german herding breeds out there