Ibn `Umar (May Allah be pleased with them) said: The Messenger of Allah (PBUH) prohibited riding a camel which eats dung, or animal or human waste.[Abu Dawud].
Al-Jallalah means an animal which usually eats filthy things including human excrement. This filth becomes a part of his body and it stinks. When such animals are disgusting for riding, they would be obviously unlawful for food. It is, however, necessary that the Al-Jallalah should have the characteristic mentioned in the Hadith because ordinarily almost every animal does eat a little amount of filth but with the difference that it is not common food, nor does it become a part of its body. In any case, it should be borne in mind that Islam has stressed purity and cleanliness and warned against filth.