I want a pet animal that, in the case of a hero barging in to confront me, I can just throw it at him and know he's not getting back up. Something like a wolverine, or a badger. [http://en.wikipedia.org/wiki/Killer_badger] Maybe a mongoose.
I suppose I could just be boring and get a mastiff, but that's not nearly so interesting. OOO! A bobcat! Or a lynx!