The heaviest breed of dog is the English Mastiff. It weighs 110 to 343 lbs and its height is normally 25 inches at the shoulder to 36 inches. It is a big strong dog who has the strength to pull 7300lbs.
Its original inhabitant is not known. According to one hypothesis this creature was first found in Central Asia (possible larger area of Iran) and in South Africa. It was only through ancient relics, drawings and sculptures discovered later that it existed in the African area well before Christ. The mastiff was used for hunting and in war between tribes or for dog fights.
Down the ages the mastiff caught the fancy of the powerful Romans who were on a conquering spree and the mastiff was extensively used in the war. The Romans on their invasion of British found this breed useful, therefore, brought it along with them.
The mastiff was not the original inhabitant of Britain. Some traders of African Descendents brought this breed when they migrated to Britain. The rich bought this breed due to its fearful size to hunt first for food and then later on when hunting was banned were kept as pets.
The extensive use of this breed in human fights, took a toll on the numbers on this breed and its number began to decrease. Some Traders began to look elsewhere to increase its numbers as it was a very useful dog. Eventually they found a breed which would mate with mastiff bitch in America. Though they succeeded genetically but found that the litter thus born were not that ferocious or as strong as its ancestors. Such puppies were lethargic and slow and lacked the killer instinct.
In the Twentieth Century this breed was basically kept as pets as there were no wars or hunting allowed by law. Thus being lethargic and slow they were kept in the confines of the house and they growled at strangers but became active when provoked. They love to be cuddled and are loved by children despite their size.
The bloodhound and the English Bull dog are the descendants of the original Old Molossus breed of Africa. But the English Mastiff is the original Mastiff.
There are other heavy breeds of dog include St. Bernard which weighs 150 to 260 lbs, Irish Wolf hound which weighs 120 lbs, and Great Dane which weighs 200lbs.