There are some dogs that breeders «choose» to leave for breeding purposes only for a number of reasons. (allstarfrenchbulldogs.com)
Recent studies confirm that dogs confined in puppy mills for breeding purposes demonstrate impaired health and diminished welfare and carry emotional scars for years. (nationalpuppymillproject.org)
As for small animals, like rats, rabbits and guinea pigs, because of their original breed purpose, you need to be very careful. (pethelpful.com)