[#1791; Colita der Res (Spanish)]
While this cut was originally the tail of a steer or ox (a bull with his balls cut off to greatly improve his disposition), today it can also be from a cow. Oxtail was once considered food for the poor, but today is quite expensive, valued for its unique characteristics.
The main use for oxtail is in soups, where it's gelatinous nature provides body. This characteristic is particularly essential in mock turtle soup, but is used in many other soups and stews worldwide. It is no doubt even eaten in India, though India likes to pretend beef is not eaten there.
Whole oxtails vary from 2 to 4 pounds each, so when a recipe calls for "one oxtail", 3 pounds is a good bet. The photo specimen was 19 inches long and weighed 3 pounds, purchased from a large Asian market in Los Angeles (Alhambra) for 2017 US $3.99 / pound. The largest piece in the photo weighed 9 ounces, the smallest 2-3/4 ounces, but pieces weighing less than an ounce are sometimes included in a tray of pieces.
Buying: It pays to keep an eye on your local meat markets, because the cost of oxtail can vary as much as a few dollars per pound. On the same day (in July 2017) I saw trays of cut tail from $3.99 / pound (on sale) to $7.99 / pound.
Prep: Because oxtail, at least here in California, is almost always sold already skinned and jointed, there is no prep. Even rinsing off saw kerf is not needed as they are generally cut with a blade.
Cooking: Cooking time is usually 3-1/4 to 3-3/4 hours. After cooking, the meat is most often removed from the bones and the bones discarded. Some recipes call for serving "on the bone", but in either case the flesh should easily and cleanly scrape off the bones. Aromatic vegetables and seasonings are often included during the long cooking, and in most cases, are discarded with the bones. The soup is then finished using fresh vegetables and the meat added back near the end of cooking. In some cases, the overcooked vegetables become part of the sauce.
Subst: The best substitutes are Beef Shanks and the much less costly beef neck bones, but they are imperfect substitutes. More tender cuts of beef definitely will not.
More on Cuts of Beef.