"An ox and a donkey together" represents the unification of the defiled spirits of the Other Side against man. This is applied to the struggle between Simeon and Chamor, a conflict in which Simeon - who was under the astrological sign of the ox - fought Chamor, of the side of the donkey, to prevent the two from joining. We learn that Simeon first brought the blood of circumcision on the males and slew them, just as God did to the Egyptians (who represent donkeys) through Mosess, when Joseph (the ox) was removed from them. In the final Exile, we're told, Jacob and Joseph will rise against Esau and prevail.
Unity is recognized as the supreme power in our world. United evil can easily defeat disunited good. Therefore, the only way to defeat unified evil is through the unification of the good and the righteous. Through the cleansing power of circumcision, this passage creates disunity and separation within the unseen evil forces lurking in our midst.