Question is pretty self explanatory, is the white stuff fat, gristle or something else?
Fat. Fat doesn't always look the same - this is the exterior layer of tough, gristle-y fat that becomes silky and amazing with a long braise. If you don't know that the white stuff is fat, I am assuming you don't know how to cook oxtails - you can't just pop them in a pan for 10 minutes and then done - they need to be long cooked with spices and you'll have yourself an incredible meal. Brown them in a pan first, take them out of the pan and add chopped onions, carrots, and fennel (or celery) to the pan with salt/pepper. Once those start to caramelize, bring back the oxtails, throw in a couple of cloves of garlic and a sprig of thyme and a little rosemary - then add some broth or wine to cover the meat and cover the pan tightly. Put it in a 275 degree oven for 4 hours then check - the meat should fall off the bone. Low and slow cooking is the key to succulent oxtails. That layer of fat mixes with the onions, carrots, and fennel to create the most amazing sauce! Omigod I think I need to buy some oxtails for dinner tonight!
Obviously it is Fat. Oxtail contains around 14 grams of Fat in 100 grams of serving where 6 grams consists of saturated fats. It also consists of Sodium, proteins and carbohydrates. Oxtail is mostly preferred as a part of stew and soup. Bone marrow in the oxtail is very useful for health and adds flavor to it. Hence it is mostly preferred as one of the ingredients in soup. Oxtail is one of the good sources of iron.
The "white stuff" is fat and connective tissues, which melt down to make a wonderful, succulent sauce. Also, if cooked long enough, there is a load of gelatine in the bones which make the sauce even more super yummy.Google braised oxtail, or oxtail stew.
Here is one good recipe - I don't add any flour to it, and use dry cider instead of rioja (The cider adds a super flavour and is loads cheaper)
And after eating the oxtail, keep the remaining bones to cook again for stock. I freeze them, then add them to chicken carcass bones for stock, or add to beef bones etc etc.