A cleft palate is a congenital defect in foals where the palate doesn’t form properly. Milk can then flow from the mouth into the upper airway and lungs, causing inhalation pneumonia and eventually killing the foal.
There is likely a teratogenic cause meaning that the mare ingests something while the foal is in utero, which causes a birth defect. It is the reason I advised all my clients to NEVER deworm or administer any medications within the first 60 days of pregnancy. I have a reason behind this advice which I will share when I get a moment.