For example, fish skin leftover from processing sea food. Cow esophagus. Pizzle. Or the little bits of meat and connective tissue left on a carcass after butchering ground up into a 'stew' along with vegetables that didn't pass muster for human consumption.
I also give my dogs ribs and knuckles, though it's not always easy to find bones at the grocer any more. Most people just want boneless, bloodless shrink-wrapped steaks, whether that's beef, chicken or pork.