The adjective pugnacious means “inclined to quarrel or fight readily”, “combative”, or “tough and callous by virtue of experience”.
Of course, pugnacious has nothing to do with pugs – they’re too sweet! The word comes from Latin pugnus, meaning “fist”. Several other English words also developed from that Latin root, including repugnant, impugn, and pugilist. See the Online Etymology Dictionary for more info about all those words.
I’m surprised I’ve never seen a pun on pug and one of those words before. If I ever get a pug, I’m totally naming him Pugnacious! I also want to have a dog with the name of another breed, like a bulldog named Poodle or a chihuahua named Great Dane. Or a celebrity name, like Elizabeth Taylor. :P