In act 3, scene 1, mercutio refers to tybalt as a “ratcatcher” and the “prince of cats.” think about all of the connotations of “cat.” what do you think mercutio is saying about tybalt?

I'd say, since cats are known for being stealthy and sneaky, that Mercutio was calling Tybalt, well, stealthy and sneaky. I think the Prince of Cats thing is also an allusion because I remember reading about it somewhere but I can't remember much (eek! sorry...) 
But yeah, I think he was saying that Tybalt is sneaky and stealthy, but also skillful in many ways, and treacherous, to say the least.

Send me the exerpt and i will read it.