This is the rectum and large intestine of the pig. In the West it is commonly used as a large sausage casing. In East and Southeast Asia it is a popular street food. They are also prepared similarly in Mexico.
If sold for cooking it's called "Pork Bung", if sold for making sausages
it's called "Hog Bung". The photo specimens were 23 inches long, 2 inches
diameter at the big end and weighed 6-1/4 ounces.
Buying:: If you intend to cook these, they're most easily found in markets serving an East or Southeast Asian community. Look for them in the frozen food lockers. If buying for sausage making you can get them from the usual purveyors of sausage casings.
Prep: If you've just butchered your own pig, you've got a good deal of scrubbing to do, and generally need to turn the bung inside out to clean the inside. If you've bought a package in a North American market, they're cleaned up and ready to go.
Cooking: Generally these are simmered for an hour, then sliced diagonally about 1 inch wide. They may be deep fried, often wrapped around a scallion, or stir fried. If fried crisp they are usually served with a dip. In Mexico also they are served with beans and tortillas if still chewy, on there own (perhaps with hot sauce) if crisp.
Yield: If you've bought a 1 pound package you should have about 8 ounces or so after simmering (50%).