Great dane is not a guard dog. Even though they are big in size, they tend to have a calm temperament. If you want to scare off an intruder with his size, its ok but dont expect from him to attack the intruder.if you want a dog which is huge and have guarding instinct,go for bull mastiff. But, all huge size breeds have one disadvantage-they have short span of iife and tend to have a lot of health problems. If im to choose a guarding breed id go for dobermann pincher. They are trainable, shed less , extreamly loyal and go along well with children. They are renowned guard dog. Jhon louis doberman, a german tax collector invented this breed to protect himself from thieves. You know, you should watch dogs 101 on Animal Planet.not sure if they are running the episode but you can watch the archives on youtube.