Final answer:
The heaviest and largest dog breeds are classified as Mastiffs or Livestock guard dogs, which have been selectively bred for pastoral purposes like herding and protecting livestock with moderated instincts.
Step-by-step explanation:
The heaviest and largest breeds of dogs are usually classified as types of Mastiffs or Livestock guard dogs. These breeds have been integral to the development of pastoralism as they were domesticated for herding and protecting flocks of animals like sheep, goats, and cattle.
The natural instinct of dogs was selectively bred into a moderated form known as eye-stalk-chase-bite, which is less aggressive and more suitable for herding rather than hunting, thereby enabling these dogs to effectively guide and protect livestock.