I believe the answer is: "A plague o' both your houses! 'Zounds, a dog, a rat, a mouse, a a cat, to scratch a man to death! A braggart, a rogue a villain, that fights by the book of arithmetic! Why the devil came you between us? I was hurt under your arm"
Mercutio said that his death happened because of a 'mouse' and a 'cat'.
I believe, these animals are a metaphor to describe two families that are always in conflict with one another.
Which mean that he is referring to Romeo's and Juliet's families, the Capulets and the Montagues