The phrase "bait dog" refers to a dog that is used intentionally as a victim in dogfighting. It is used to describe a dog that is used to train other dogs to fight by being attacked or used for practice. Full definition
I also told them that the dogs given away free are sometimes sold to labs for testing on, or used as bait dogs for fighting. (okcpetsmagazine.com)
Do you agree that the dog will not be used as a guard dog, or dog fighting bait dog, or be used for breeding or experimentation purposes? (midwestbtrescue.org)
Her injuries proved to be those found commonly among bait dogs, or those used to train aggression in fighting dogs. (onegreenplanet.org)