There are actually three songs in Animal Farm. The first is called "Beasts of England," which the animals learn from Old Major in chapter one. It is an inspirational tune that envisions a future golden age when the "tyrant" humans will be overthrown. In it, the animals hear of harnesses, bits, and spurs left to rust and of receiving an abundance of food, such as oats, wheat, and hay. The song is similar to the Communist anthem called "The Internationale."

The animals sing the inspiring "Beasts of England" until the pigs replace it with another anthem in chapter seven. This one is called "Animal Farm!" and has less rousing lyrics. It is also much shorter, consisting of only two lines:

Animal Farm, Animal Farm,
Never through me shall thou come to harm!

Finally, Napoleon replaces this second song with yet another, called "Comrade Napoleon" in chapter eight. It says nothing about either a golden future or Animal Farm; it is entirely about loyalty to Napoleon.

The song you are referring to is called "Beasts of England." It was taught to the animals by Old Major in chapter 1 of the book.

I will sing you that song now, comrades. I am old and my voice is hoarse, but when I have taught you the tune, you can sing it better for yourselves. It is called 'Beasts of England'.

When Old Major speaks to the animals, he inspires them with the vision of a world without man that he saw in a dream; then he tells them that they must never falter in their belief in the cause of equality for all animals. He gives them a song to sing to inspire and support the rebellion.