American Bulldogs descend from working bulldogs that settlers brought to the South, where they caught loose cattle and feral hogs and guarded farms. The type nearly disappeared after World War II until John D. Johnson and later Alan Scott revived it. Two types came out of that effort: the heavier Johnson, or bully, type and the leaner Scott, or standard, type. Many dogs today are a mix of both. Adults weigh roughly 60 to 115 pounds and are usually mostly white with colored patches.
That farm background explains the stamina and the protective streak. Most are devoted to their people, but they're strong enough that early socialization, reward-based training and a secure leash and fence matter from the first week. Many shelter dogs labeled American Bulldogs are mixes, so size, energy and coat vary from dog to dog.