Cassius uses animal imagery here to paint a withering picture of the mass of Roman people whom he sees as blindly following Caesar, and also to undercut Caesar's standing. He describes the Roman people contemptuously first of all as 'sheep' and then as 'hinds', or deer, while he likens Caesar to a 'wolf' and 'lion' in relation to the people. Thus he pits the image of strong, fearsome animals (the wolf and lion) against the timid, the feeble, and the easily led (sheep and hinds). The use of the word 'hinds' is especially significant as it refers to female deer, so that Cassius portrays the Roman people as being womanly, which he equates to being weak.
Although Cassius' use of metaphor here is obviously aimed at the Roman people, he also uses it to subtly undercut Caesar. In effect, he is saying that Caesar only appears strong and powerful alongside weak fools like what Cassisus believes the Roman people to be. We have seen already that Cassius has a very low opinion of Caesar, considering him to be weak and unmanly. Cassius deliberately plays up his weak points, such as his epilepsy and the fact that Cassius was obliged to save him from drowning once. Cassius is utterly scornful of the fact that such a man has grown to a position of such power in Rome. Cassius is, of course, envious of Caesar's phenomenal rise and is bitter that he himself has not gained the honours that Caesar has. He considers himself the better man and deplores Roman society for allowing Caesar to ascend to the top. He manipulates others, particularly Brutus, into believing that Caesar must be eliminated for the greater good of Rome. Brutus is inveigled into joining the conspiracy against Caesar, not because he despises Caesar personally (quite the opposite) but because he thinks that no one man should be allowed to have so much power and run the risk of becoming a tyrant.
In lines 103-5 Cassius says,
' I know he (Caesar) would not be a wolf
But that he sees the Romans are but sheep:
He were no lion, were not Romans hinds.'
The metaphors compare Caesar first to a wolf and then to a lion, the Roman people first to sheep and then to hinds (deer). The sense of both sets of metaphors is the same: Caesar acts like a tyrant only because he sees that his people are weak and allow him to do so. Cassius uses metaphor as part of his rhetoric to convince Casca to join the conspirators.