Compare goats and sheep: How is Muriel the goat different from the sheep in Animal Farm?
The only goat in the story is Muriel. She is described as "the white goat" in chapter 1. Unlike the sheep, Muriel is intelligent and can read.
Muriel, the goat, could read somewhat better than the dogs, and sometimes used to read to the others in the evenings from scraps of newspaper which she found on the rubbish heap. (ch 3)
Muriel is independent, whereas the sheep are described en masse. Muriel is also kind and motherly, but brave. She even participates in the battle, and she and Benjamin (the donkey) pull a cart during the harvest. Muriel is a fervent supporter of the rebellion. She is upset when the song Beasts of England abolished. The other animals often ask her to read the commandments when thet realize that they have changed.
Muriel is very important to the story in a symbolic way. She symbolizes the rebellion from the common animal's standpoint. She is always the first one mentioned when something changes.
The sheep are described in groups, not individually. They are referred to as some of "the stupider animals" like chickens. The sheep are used by Napoleon to drown out the voices of dissent during the meeting ("four legs good, two legs bad" and "four legs good, two legs better") and make the other animals feel that they are weaker by sheer numbers. The sheep are the first to falsely confess.
Then a sheep confessed to having urinated in the drinking pool--urged to do this, so she said, by Snowball--and two other sheep confessed to having murdered an old ram, an especially devoted follower of Napoleon, by chasing him round and round a bonfire when he was suffering from a cough. They were all slain on the spot.
The sheep represent the common people, and those who are easily manipulated. They go along with what they are told, and they are easy to sway.