Using animal imagery allows Morrison, the author, to vividly describe how slavery dehumanizes a person. The slaves are treated worse than animals in some cases. The schoolteacher characterizes slaves as just another kind of animal, listing what they have in common with animals. He believes you have to "handle" slaves just as you do cattle or dogs. Paul D. looks at the rooster and thinks it has more freedom than he does because it is allowed to stay what it was born to be -- a rooster. Paul D. says as a slave, he hasn't been allowed to remain human, that whatever it is that made him human has been taken away from him through slavery.
Toni Morrison goes on in Beloved to describe the effects of slavery on both the slave owners and the enslaved. Slave owners become monsters in order to treat slaves like animals. One of the cruelest effects she shows it how slavery separates mother and child and tries to destroy the power of a mother for her child. Sethe's maternal instincts are so twisted that she tries to kill her own children rather than let them realize what it is to live as a slave.