Oxtail, which is the tail of a steer cut into smaller portions, is gelatin-rich due to the large amount of collagen. When cut the marrow and bone are in the center surrounded by meat and fat.
Previously, oxtails were used mainly for stews and soups but now many chefs have come up with a variety of dishes to serve oxtail.
Either way, long slow braising in a liquid is the way to go!