The Neapolitan Mastiff (Mastino Napoletano, in italian) is a natural guard dog. Their protective instincts and loyalty to the family are strong characteristics that make them fearless and extremely dangerous to strangers.
But when you're abble to socialize with those dogs, like I fortunately did, they can be very sweet and playful. Although you must be careful when playing with a Mastiff, since they weigh between 60 and 90 kg. They are also known for having the largest cranium of all dog species.