Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Prophet Muhammad (peace and blessings be upon him) forbade drinking the milk of Jallalah (animals that eat filth), and from Mujaththama, and from drinking directly from a water-skin by putting the mouth to it.
(Imam Ibn Hibban, may Allah have mercy on him, says:) Jallalah refers to that animal whose main food is filth, but if most of its fodder is pure things, then it will not be considered Jallalah.