The misconception that the Miniature Pinscher is a "miniature Doberman" occurred because the Doberman Pinscher (a breed developed by Karl Friedrich Louis Dobermann around 1890) was introduced to the US before the Miniature Pinscher. In 1919 the Miniature Pinscher was introduced to the AKC show ring. At the time, not knowing that it was referred to officially in Germany as the Zwergpinscher (little biter), the AKC referred to the breed as simply "Pinscher" and listed it in the miscellaneous category. When the Miniature Pinscher Club of America (MPCA) was created in 1929 (the year of the breed's official introduction into the AKC), they petitioned for Miniature Pinschers to be placed in the Toy group. The AKC's description, that the dog "must appear as a Doberman in miniature", led to the misconception common today that this breed is a "Miniature Doberman Pinscher".

The original name for this breed in the US was "Pinscher (Toy)" until 1972 when the name was officially changed to Miniature Pinscher.

More Info: en.wikipedia.org