I had a dog at the front of my fort entrance tunnel. He was only there to be an early warning system for my dwarfs to shut the floodgates, but he did fairly well. The first kobold who came was killed by him, but he took some damage as well. For sake of convenience I'll call him Dog.
So, the first battle he fights the kobold to death, but he broke his front right leg and got minor brain damage. I didn't figure he would last long, but he wasn't meant to. He was just meant to give the warning out and die.
Second battle was a baby snatcher. He destroyed it. Mangled and ripped limbs off and no damage to himself. I was pleasantly surprised.
Third and fourth battle. Two kobolds a couple days apart. First one got away from Dog, lost his left leg. He then died of thirst later. Second kobold comes and promptly dies. Still no further damage to Dog.
Fifth battle. Baby snatcher comes. Dog exacts karma and rips both it's arms off. He ran and got away, but he wont be stealing babies again. ^_^ Still no damage to Dog.
Sixth through tenth battle. Goblin ambush! First one of the game and a mere four goblins. Dog kills 1, maims another and the last two retreat. Dog still has no damage.
Eleventh battle. My miners uncover the underground river parallel with my valley entrance. They the skip off and leave the inhabitants to poor Dog to take care of. I send my mason to fix the wall, but one olmman gets through. The mason runs in panic, and leaves it to Dog to kill. Dog kills the olmman, but is maimed in the upper body and dies of blood loss.
I felt kind of bad that he died. I had set him there to just be a early warning system, but he had proved a good fighter. Too bad he was incapable of walking or I would have assigned him to my real military. As it is, he died a rather anti-climatic death.