Read the first stanza and think.
(i) Is Macavity a cat really?
(ii) If not, who can Macavity be?
(i) No, Macavity is not a cat. The cat is just a representation of the master criminal.
(ii) The poet did not mean Macavity as a cat. He just has given a poetic form using the cat to refer a criminal who is ferocious and very intelligent one. He commits the crime in a cunning quiet manner as a cat does.