Season: Place the oxtails in a large bowl. Rub with mustard. Combine the dry ingredients in a small bowl. Then, rub all over the meat. Cover and refrigerate overnight.
Heat Grill: Heat your smoker to 250F degrees with apple, cherry and/or hickory chips with an indirect heat zone.
Make Spritz: Combine the apple juice and vinegar in a spray bottle.
Smoke: Smoke the oxtails for about 5 hours, spritzing every 30-45 minutes with the apple juice and vinegar. Cook until they are fork tender.
Rest: Remove from the smoker and let rest 15 minutes before serving.
If you can't find the beer and brat mustard, you can use yellow or Dijon mustard.I recommend marinating these overnight, but if you don't have time, you can let them brine in the fridge for 1-2 hours, and they'll still taste great.