The slave-catching dog that was infamous in the South for tracking down its victims and savagely mauling them was the Cuban bloodhound.
It was most likely derived from the Spanish war mastiff (a derivative of the Alaunt) with maybe a tough of greyhound or scent hound blood.
Drury has a depiction of this dog:
Note how similar this dog is to the dogs in “The Hunted Slaves” by Richard Ansdell.
These dogs were usually just called bloodhounds in the US, and their infamy spread through the world.
Now, the actual bloodhound of Europe received some of that bad reputation. That breed of bloodhound is a pack hound. It is quite docile and gentle. It has a great nose, and if someone can stand living with a rather large, active dog that is driven more by its nose than a desire to please its people. (These dogs are not “Ol’ Duke” from the Beverly Hillbillies. They do require a lot of exercise.)
So when one reads accounts of bloodhounds savaging people, especially escaped slaves trying to escape the oppression of their captors, one must remember that the bloodhound in question is the tracking mastiff from Cuba.
Natural History 
[…] is a pack hound. It is not an aggressive breed at all. He may have been confusing them with the Cuban bloodhound, which, as far as I know, never existed in […]
[…] In the nineteenth century bloodhounds were depicted in literature as the ferocious man-trailers who savaged slaves as they tried to escape captivity. In reality, the writers of that day were writing about an entirely different sort of bloodhound, one that had developed in the tropical parts of America by crossing the Spanish hound with some kind of mastiff. The most famous of these dogs was the Cuban bloodhound. […]