Unlike many of its relatives, whose histories seem to get cloudier with time, the American Bully's history is extremely well-documented. It was largely developed between 1980 and 1990 by several enthusiasts who wanted to refine similar current breeds into a great family/companion dog with a distinct look.
The heart of the breed's lineage is split between the American Pit Bull Terrier and the American Staffordshire Terrier, but it's also been influenced by the French Bulldog, English Bulldog, Olde English Bulldogge, and American Bulldog as well.
Recognized by the United Kennel Club, this breed comes in 4 types, each characterized by its height, weight, and overall appearance.
The Pocket version is the shortest, with males being under 17" and females under 14", but still packs a considerable amount of muscle.
The Standard is exactly as it sounds — the standard of the breed, sitting between other weights/sizes — and is basically a larger version of the Pocket with more height and weight yet sharing a highly similar look.
The Classic is a step in a different direction, largely being the same height as the Standard but with considerably less muscle mass, having a closer resemblance to an American Pit Bull Terrier with a lighter, more athletic frame.
The XL is basically a taller, heavier version of the Standard.
Many do consider there to be more categories beyond this, such as the XXL (Extreme) and Micro, but they are currently unrecognized in any official capacity. In any case, regardless of their size, they all share a similar temperament and overall characteristics.
Earlier in their development, there was a much larger array of physical characteristics within the breed, causing the first group to recognize them, the American Bully Kennel Club, to penalize them harshly in early showings and forced handlers and breeders to tighten their efforts closer to the breed standards. Since the breed's inception, they have been rising in popularity due to their distinct looks and great personalities and were even recognized by the United Kennel Club in 2013.