Friend, the mucus is a sign of inflammation in the large intestines. The black feces is bleeding in the upper gastrointestinal tract. This can be caused by many different medical conditions. It will not be possible for me to pinpoint the exact underlying cause without a hands-on examination and some testing, I am sorry. But, common causes are ingestion of a foreign body, infections, or even part of a systemic disease to name a few causes. I would recommend a hands-on examination with your family veterinarian and x-rays as an initial screening test. In the meantime, this is what you can do:
1- Do not give things like Pepto Bismol or Imodium.
2- Change his diet to rice (75%) and boiled boneless chicken or ground beef (25%) until the stool is back to normal then wean him back into his regular diet.
3- Supplement with a probiotic, these are friendly bacterias that help with regularity and inflammation. You can buy this over the counter in any pet store.
4- Follow up with your family veterinarian for a hands on examination and x-rays as an initial screening test.
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