The idea of Animalism (Communism) is that all members of society will work together for the great good of the community. In order for this optimistic yet unrealistic method to work is if all citizens can respect and trust one another. Old Major's speech is rudely interrupted by a rat who race across the barn, as it's being chased by a dog. The rats, therefore, represent the shady element of soceity such as the criminals. The existence of the rats raises a glitch in the funamental thinking of Animalism in that not all animals can be trusted. Still, when put to a vote, the rats and rabbits are included as comrades probably because the optimists felt they could be tamed. Of course, the rats and the rabbits become the least of the animals' worries after the revolution.
It was obvious that even the other four-legged animals of Manor Farm were not fond of rats. Old Major's speech was interrupted when four large rats emerged to hear the boar's words. When the dogs saw them, they chased them back to their holes. Old Major put it to a vote. Should the wild animals, such as rats and rabbits, be friends or enemies? Although their were four against the rats--by three dogs and the cat--all of the other animals voted in favor of keeping the wild animals as friends.