AKC: http://www.akc.org/dog-breeds/nova-scotia-duck-tolling-retriever/

These Mastiff-type dogs stem from animals taken to Germany by Roman soldiers as they marched across Europe. Used to guard livestock, they were either discarded as the cattle were eaten or were left to guard outposts, and many finished up in Switzerland while others reached southern Germany. They were especially known around the town of Rottweil, which for 1,800 years was a centre for livestock trading. The evolving dog became a butcher’s dog, drover and draught dog.

The Rottweiler, which first appeared in Britain in 1936 and was shown at Crufts the following year, is an above average-sized, very agile, black-and-tan dog. Extremely strong and imposing, he is easily obedience trained and is, in fact, a dog that enjoys working. He has natural guarding instincts, but is not aggressive by nature. His expression is tranquil and kind, but when aroused, he will hold his own with any opponent. He is not a dog for the inexperienced and has been much maligned in recent years, when the breed became over-popular, and Rottweilers were often purchased to feed a macho image.