Deuteronomy 22:10

Ver. 10. Thou shalt not plow with an ox and an ass together— We refer to Lev 19:19 respecting these unnatural mixtures. This, as well as the other particulars, is thought to have respect to some idolatrous custom of the Gentiles, who were taught to believe, that their fields would be more fruitful if thus ploughed; for it is not likely that men would have yoked together two creatures so different in their tempers and motions, had they not been led to it by some superstition. This prohibition is supposed to extend to other animals of different species. Or rather, it prohibits the yoking clean and unclean animals together.