Interesting question. Here are my thoughts:
1. You have to choose an animal that is available to you. It might be a neat idea to have a lion guarding your front door, but in most communities, a dog is about the wildest animal you can find.
2. You need an animal that you can train to protect you. This means they need to be able to learn to guard their territory, attack bad people, and not attack good people. Another reason dogs win out over lions! ;)
3. You need an animal that will raise the alarm, not just attack silently. This would be why poisonous snakes are not good guards.
4. The animal needs to be capable of stopping a bad guy, not just raising the alarm. A parrot might crow loudly, but probably couldn't knock the invader to the ground and keep him there.
5. The animal needs to be legal where you live. If your favourite breed of dog is known for mauling young children, there is probably a law against your having one.
I think these guidelines are quite reasonable. You want to have an animal capable of protecting you, but not one that will hurt you or wrongly hurt (or OVER hurt) anyone else.