APBTs were not originally bred for fighting. The APBT is a bulldog, a descendant of the original bull-baiting and bear-baiting bulldogs of England.
Bull baiting was developed around the job of the bull dog. If a bull got away from the hunter ,the bull dog would go after the bull and hold it down by it's nose until the hunters arrived for the bull. From the bull dogs history of being able to grip, the bulldog was suited for this work, and he now started to hold the animals for the butcher ,instead of the hunter. These are where the first gripping dogs derived from. This is how bull baiting became a sport.
When bull-baiting and bear-baiting became outlawed in England, the dogmen began breeding bulldogs specifically for pit fighting. Breeders brought their animals to the New World, and the rest is history.
Supervision of this breed around other animals is a must. Yes, the American Pit Bull Terrier does live in harmony with other animals, but because of their pit-fighting genetics, they must remain under close supervision. My Sadie and Tigger live in harmony together, but they are never left alone together. If no one is home, Sadie goes into her crate and Tigger has the run of the house. I would hate to come home to one hurt and another very dead dog because I was careless!
Never trust a pit bull not to fight. Pit Bulls fight like no other dogs; from years of selective breeding they have a trait never to quit until the other dog is dead or very hurt. In the heat of battle they will not quit.
Pit Bulls are not human-aggressive. The breed is hard-wired for dog-aggression, through hundreds of years of selective breeding by dogmen. During this time, any human-aggressive dog was culled, therefore creating human-friendly dogs from this selective breeding that were also fiercely aggressive towards other dogs. No dogman would want to have a human-aggressive dog in the pit fighting ring, and then have to handle the dog. These dogs I would consider to be most trustworthy around humans.