(i) Is Macavity a cat really? (ii) If not, who can Macavity be?
(i) No, it's not a cat Macavity. The cat is just the master criminal's depiction. (ii) The poet wasn't talking about Macavity as a cat. He has just provided a poetic form to use the cat to refer to a fierce and very smart criminal. As a cat does, he commits the crime in a cunning, quiet way.