
Googling seems to suggest they are a primitive form of Bulldog bred in the Southern States for holding wild hogs and cattle, and for guarding.
So we are talking a highly driven determined dog which won't back down.
Temperament or lack of seems to be a big issue in the US and they are a breed like then APBT often involved with poor ownership and horrendous attacks on dogs an people are not uncommon. they are not recognised by the AKC or the other show organisation the UKC in the states.
This link:
http://www.yourpurebredpuppy.com/reviews/americanbulldogs.html makes worrying reading especially:
If you don't want to deal with...
A large dog who takes up a lot of space in your house and car
Vigorous exercise requirements
Rowdiness and exuberant jumping, especially when young
Providing enough socialization so their protectiveness doesn't become aggression
Aggression toward other animals
Strong-willed mind of his own, requiring a confident owner who can take charge
Shedding
Slobbering and drooling
Gassiness (flatulence)
Serious health problems
Legal liabilities (public perception, future breed bans, insurance problems, increased chance of lawsuits)
An American Bulldog may not be right for you