Sicilian donkey or Equus africanus asinus

The ridges on the underside of a mule's mouth are called oral or palatal ridges and are neither cartilage nor enamel. They're typically soft tissue structures made up of mucosa, the moist lining inside the mouth. These ridges are part of the oral anatomy and are covered by the mucous membrane to help with the donkeys' ability to grip and process food effectively.