How does Orwell vividly portray the importance of the sheep and the dogs in Animal Farm ?

The dogs symbolize the military/police. Initially they supported eliminating any consideration of the rats or rabbits as potential comrades, but after the success of the revolution Napoleon removes nine puppies from their mothers to be trained by and for himself.

The dogs are trained for protection and are used as Napoleon's bodyguards. Their presence stymies any questioning of Napoleon's control.

The sheep represent the uneducated, easily manipulated masses. They cannot think for themselves and happily allow others to think for them. Their acceptance of Napoleon's dictatorship and consistant repetition of his slogans drown out all challenges to his leadership.