The demonology talent 'demonic resillience' reduces the damage to a pet by 15%, but I'm not aware of a 'more armor' talent. If they pick on your pet, fear them and dot them. If they pick on you, have your pet eat them (while you die... if they are a rogue).
It also depends which pet you have out. I'm demo spec right now and use felhunter or feln00b in pvp... depends which bg I'm in.