Benjamin is the old, irritable donkey who doesn't care who's in control, because he says that work will be the same either way. He is one of the few on the farm who can read, and who still remembers the original commandments. He has an unlikely friendship with Boxer as well. Does Benjamin's character represent a real figure, person, or group of people?
It's unknown for sure, but it most likely refers to either:
The Mensheviks, a non-communist party in Russia, because he's another viewpoint from the pigs but is mostly ignored.
He also could be the older population of Russia, because he is old and skeptical.
He could also just be the skeptical people, who don't believe communism will really help them much.
He could be the Jews, who weren't helped really at all by communism.
Finally, he could be Orwell (inserted into the story, not really fitting the allegory). Just along for the ride, not really in favor of communism/the pigs either.