As somebody has already mentioned a dog. Whilst on the move my 8st German Shepherd can be quite rude to strangers. Youngsters invariably ask "Does he bite?" I always answer "Yup"
After my boat was broken into on its mooring, nothing stolen but hundreds of ££'s damage to doors and frames, I simply stopped locking it, but hung a neat notice over the door that said: -
Inside is a booby trap made from a strong spring and razor wire, you will bleed it will hurt
Not been broken into since, neighbours either side have though. One day I'm afraid I'll forget to disarm it before I enter, but that will be my fault.