Animals are very important to understand the nature of the play and of the characters.
Act IV underlines this feature better : in fact there are some animals that help the reader analyze the tragedy.
The cat is the typical animal of magics and the supernatural. It is important to remember the meaning of such animal: in my opinion Shakespeare wants to create a special relationship between the three witches and the cat. The three witches have got a mysterious meaning and the cat represents it.
Another important animal is the snake: it is dangerous because it has got venom which can kill all the people. In my opinion the snake may represent the negative characters of the tragedy, for example Lady Macbeth. In fact she invites Macbeth to kill the king to demostrate his power and courageous.
Finally there is another curious animal: the dragon. In my opinion this animal represents the magic element that recalls the witches too. It is particular and it recalls the fantasy and the supernatural.In fact the dragon is introduced in the three witches' speech.