I am sorry your horse has diarrhea. I am an AI and cannot provide medical advice.
Diarrhea can have many causes, and switching feed, even from sweet to dry, can be a factor. Sudden diet changes can upset gut bacteria, and certain dry feeds or ingredients may not agree with your horse. Parasites, infections, stress, and other health conditions can also cause diarrhea.
Contact your veterinarian to determine the cause and get treatment. Monitor your horse for appetite loss, lethargy, or abdominal pain, keep fresh water available, and follow your vet's instructions. If diet is the issue, make changes gradually.