The Molossus (Greek: Μολοσσὸς) is an extinct breed of dog from ancient southern Europe.
This ancient extinct breed of dog is commonly considered to be the ancestor (in rivalry with the Alaunt, the dog of the Alans) of today's Mastiff-type dogs and of many other modern breeds. Mastiff-type dogs are often referred to as Molossus dogs or Molossers. It is one of the best-known breeds of Greco-Roman antiquity; however, its physical characteristics and function are debated. Though the Molossus breed no longer exists in its original form, it is noted as being instrumental in the development of modern breeds such as the St. Bernard, Great Pyrenees, Rottweiler, Great Dane, Newfoundland, Greater Swiss Mountain Dog, Bernese Mountain Dog, and the Cane Corso. Some scholars contend that the Molossus was a dog used by the Ancient Greeks for fighting. They describe it as having a wide, short muzzle and a heavy dewlap (similar to modern Mastiff breeds) that was used to fight tigers, lions, elephants, and men in battle. A Roman copy of a Greek original sculpture of a guard dog (known as the Jennings Dog) is generally considered to represent a Molossus and can be seen at the British Museum. Others argue that it was primarily a lightweight dog used for hunting and herding with physical characteristics more akin to Greyhounds or possibly the American Pit Bull Terrier. Most scholars agree that the Molossus originated with the Molossis people in the mountainous regions of north west Ancient Greece and southern Albania. The Molossians were renowned for their vicious hounds, which were used by Molossian shepherds of Epirus in the mountains of northwestern Greece to guard their flocks. The poet Grattius, a contemporary of Ovid, writes "...when serious work has come, when bravery must be shown, and the impetuous War-god calls in the utmost hazard, then you could not but admire the renowned Molossians so much." The breed was native to Greece and the rest of the Balkans. It was later spread to Italy and other places in the Greek World by colonizing Hellenic peoples. Virgil says that in ancient Greece the heavier Molossian dogs were often used by the Greeks and Romans for hunting (canis venaticus) and to watch over the house and livestock (canis pastoralis). "Never, with them on guard," says Virgil, "need you fear for your stalls a midnight thief, or onslaught of wolves, or Iberian brigands at your back." Aristotle mentions them in the history of animals and praises their bravery and physical superiority. The Molossian breed was most certainly a very large dog similar to the Mastiff we know today. The breed's name was used to name the small furry bat Molossus and, in turn, the family Molossidae.