When Macbeth sees the soldiers coming toward him bearing branches from Birnam Woods, he realizes he has been tricked by the witches and now is trapped. He likens himself to a bear that is chained to a stake to be tormented and killed. Nevertheless he says that
But, bearlike, I must fight the course.
He later states that he will fight rather than be "be baited" by the "rabble's curse."
Likening himself to a bear being cruelly baited shows that Macbeth is no longer taking responsibility for what he has done. He is not and never has been a helpless animal, no matter what he might think now of what the witches have done to him. He freely chose the path of murder and mayhem, and now is paying the price of his own actions. However, he is accurate in comparing himself to an animal in that he has been dehumanized by his experience as illegitimate king.
Macduff likewise compares Macbeth to animals, but not normal animals. He likens his foe to mythological creatures such as a "hell-hound" and "our rarer monsters." While he exaggerates to a degree, Macduff is also not wrong: Macbeth has turned into a monster by the end of the play.