What caused the excitement among the farm animals?

Old Major is sure that he has, in his dream life, uncovered an old animal anthem that has lain dormant for generations. It is called “Beasts of England,” and he sings it to the other animals. Orwell describes the song as “a stirring tune, something between Clementine and La Cucaracha” (32). The song glorifies the freedom and joy that will follow “Tyrant Man’s” overthrow, and he urges all animals to “toil for freedom’s sake,” even if they die before the cause is won.

The song rouses the animals, even the dullest of whom learn it in minutes. In fact, the animals are so taken with the song that they sing it five times in unison.