Technically speaking, Animalism did not actually exist before the revolution. But Old Major had discussed the major ideas of Animalism in his speech. The idea that man actually could be overthrown was new to the animals, as was the thought that all of their hard labor could someday be for their own benefit, rather than man’s. But it’s important to note that the animals found it difficult to get the revolution started until they became desperate with hunger.
The rebellion finally happens because the animals are being mistreated, and most importantly, underfed. Although Old Major had inspired the animals to work toward an eventual overthrow of the human race, it didn’t actually happen until the animals reached a level of hunger that they hadn’t reached before.