Napoleon would not allow the singing of “Beasts of England,” because it was associated with the rebellion and the rebellion was over.
Old Major developed an interesting song that he sang at the meeting when he told the animals about his dream of them controlling their own fates.
As he had said, his voice was hoarse, but he sang well enough, and it was a stirring tune, something between Clementine and La Cucaracha. (ch 1)
The animals learn the song, and sing it at the secret meetings before the rebellion. After the rebellion, they sing it at the flag ceremony. The song even spreads throughout the land, to animals on other farms.
When the “traitors” were executed, the animals began to sing the song. Squealer announced that Napoleon had forbidden the singing of the song.
`It's no longer needed, comrade,' said Squealer stiffly. `Beasts of England was the song of the Rebellion. But the Rebellion is now completed. (ch 7)
The animals are saddened by the fact that they can no longer sing the song, and its banning is an example of how the pigs have taken the place of the humans as oppressors, and in fact the rebellion only succeeded in replacing the humans with the pigs.