I use apple cider vinegar on top of my dogs food, diluted with water. I may add some olive oil or blackstrap molasses to hide the taste. My dogs coat shines within hours. It is lush and soft.
My chickens look shiny with a dash of ACV in their drinking water.
Our Tibetan Mastiff/German Shepard mix puppy had really itchy dandruff and a really dull coat. I started feeding her about 5ml of apple cider vinegar once or twice a day. She really doesn't like it, and when I add it to her food, then she doesn't want to eat her food. Instead I put it in a syringe and feed it to her that way. Her coat improved in about two days, and now it is thick and shiny and not much dandruff. She also had weepy eyes and now she doesn't. Other dogs from the same litter have thin hair and patches coming out. Our landlord was so impressed that they started using it themselves! They said that our dog looked like a different dog altogether.