Mastiffs are extremely protective of their owners and it gives people the impression they are aggressive. Same with the Cane Corso. Both dogs are more defensive then aggressive, but the potential is still there especially when owned by people who easily dump dogs. You know they didn’t bother training them which is the main issue

Mastiffs have historically been guard dogs, protecting homes and property, although throughout history they have been used as hunting dogs, war dogs and for blood sports, such as fighting each other and other animals, including bulls, bears and even lions.

Dunno what you define as aggressive but they definitely were bred to be.