I have an 18 year-old pony ex-stock horse. Recently, he has been having difficulty chewing.

What you are describing are very common clinical signs of an aged horse with dental problems. A complete physical and dental exam is recommended — in order to diagnose and demonstrate to you what the problems are. Only then can the dental practitioner discuss treatment options available and embark upon treating him with the method that you agree to. The packing of feed up inside his cheeks is called "quidding". Usually it is associated with periodontal disease — which means he'll have infected and inflamed gums, which could be seriously affected health of other parts of his body. X-rays may be necessary to fully evaluate the condition. The packing of feed up inside his cheeks is called "quidding". Usually it is associated with periodontal disease — which means he'll have infected and inflamed gums, which could be seriously affected health of other parts of his body. X-rays may be necessary to fully evaluate the condition.