We also had a cat a few years ago that was better than any guard dog, if someone came to the house or garden that she did not recognize she would go on the attack.
Strangely we also had a little black rabbit who did the same, especially did not like the postman, it's name was Flop when we got it but the kids soon renamed it Psycho Bunny.
It is strange how the un-likeliest of animals can become very defensive

Cheers

NCM