Perhaps one of the working dogs that are steadfast but will also go kind of nuts if prevented from doing their duty?

Okay, yeah, I totally buy it. Not something classic like a German shepherd or Doberman, but something like a Sheltie (incredibly stubborn, herds everything in its vicinity, adores kids, no one can stop it from retrieving a lost lamb even if the "lamb" is actually a rock that it has fixated on for some reason) or a terrier (unshakably loyal to everyone it decides is its own, will bite anyone else's ankles, unquestioningly obeys every order it agrees with and ignores all others).