Do not be mismated with unbelievers. For what partnership have righteousness and iniquity? Or what fellowship has light with darkness? (2 Corinthians 6:14 RSV),
Mismated is literally a term that means unequally yoked.
A yoke is a wooden frame or bar with loops at either end, fitted around the necks of two animals, which tied them together and forced them to function as one. That is what Paul speaks of here. He is thinking of Deuteronomy 22, where the Law says, Do not plow with an ox and a donkey yoked together
(Deuteronomy 22:10). That may seem strange to us, but God was concerned that the Israelites not tie together two animals of a different nature.
I have never seen an ox and an ass yoked together, though once when I was traveling in the Middle East I saw a farmer plowing his field with a camel and a donkey. It was almost ludicrous to watch. The camel was three times the height of the donkey, and its legs were three times as long. It was striding along at a rather slow pace for a camel, but the little donkey was running as fast as it could to keep up. The farmer kept beating the donkey all the time, trying to get it to keep up. It was cruel.