I also found this explanation on the internet about these kind of dogs...."The rat terrier is a famed farm helper and companion. As their name implies, they were excellent ratters on farms and made fantastic hunting companions.
They are a more rare breed today because the breed died out quite a bit due to the invention of pesticides in the 20s - 40s used on farms.
They are often mistaken for a Jack Russell Terrier, although they never have a wiry coat, they are finer boned, have a more refined head, and are considerably less aggressive.
All-in-all, they are typical terriers, loving to explore and tear around the yard, but they also love to lounge around with their owners."