Rhetorical devices are persuasive devices. Old Major wants to persuade the other animals that they can have a better future. He wants them to believe they don't have to accept their lives of endless toil.
To do this, he first appeals to his own ethos, or credibility. He reminds them of his great age and that he has had much time to think. These attributes give him authority. He says that for these reasons:
I understand the nature of life on this earth as well as any animal now living.
Building his own authority makes it easier for him to persuade the other animals to accept what he has to say.
Old Major also uses pathos, or emotional appeal, when he says he doesn't think he will live much longer. This encourages the other animals to pay closer attention to his words, because they might not have access to his wisdom much longer.
Old Major leans on logos, or logical appeal, as well. This is the persuasive power of facts and statistics. He states that the resources are there in abundance to support a better life for the animals:
This single farm of ours would support a dozen horses, twenty cows, hundreds of sheep
Old Major persuades, too, by using hyperbolic or emotionally weighted rhetoric when he describes the animals' current plight to appeal to their emotions. They are "forced to work to the last atom of … strength" and treated with "hideous cruelty."
Old Major also employs oversimplification in employing what is called a "black and white" view of the world to make his case. It is either them or us. Humankind is bad and the animals are good. Old Major states:
Man is the only real enemy we have. Remove Man from the scene, and the root cause of hunger and overwork is abolished for ever.
As the novel will show, "Man" is hardly the animals' only enemy, but Old Major's rhetoric gives the animals a common enemy to unite around.