They are a bull/terrier mix
“A dog (Olde English Bulldog) that looked much like today’s pit bull was originally used in the 1800’s in the British Isles to ‘bait’ bulls. These matches were held for the entertainment of the struggling classes; a source of relief from the tedium of hardship. In 1835 bull baiting was deemed inhumane and became illegal, and dog fighting became a popular replacement. Soon, a new bulldog was created by crossing the Olde English Bulldog with terriers to create smaller, more agile dogs.”
http://www.badrap.org/breed-history
Many, many other sources of information available.