The phrase "bait bulls" refers to bulls that are used as bait in the practice of dogfighting. They are meant to provoke and excite the fighting dogs before a fight. Full definition
The breed was the product of a cross between bull dogs — used for baiting bulls — and terriers, a somewhat larger, family oriented dog, creating a mix that was loyal to humans and vicious with other animals. (northcoastjournal.com)
The sport of baiting bulls was widely spread in England until 1835 when it was finally abolished. (snowyswan.com)
The bulldog, for example, was bred to bait bulls back in England, but now it's a very biddable companion. (whatismydogtype.com)