The animals had no means to stop the pigs from becoming dictators.
Explanation:
Let's first remember that the pigs were the only animals that could read and write well. They also had access to information through the radio they had in the farmhouse. Personally, I believe that the other animals couldn't have stopped them because they were naive and had no idea that the pigs had planned to take over the farm and to become worst than the worst dictator ever.
In addition, even though there were plans for animals to learn to read and write, they never thoroughly learned to do so. A few animals were able to read a few words or scribble a few letters, and the pigs took advantage of that changing their manifesto every time they felt like it.
