"The ox of your foe" — who is the "foe" the Torah refers to? The Mekhilta records multiple interpretations. In one reading, the idolators of the nations are called "foes" of Israel throughout Scripture. (Deuteronomy 23:10): "When you go out as a troop against your foes." (Deuteronomy 21:10): "When you go out to war against your foes." The word "foe" in these military contexts refers to non-Israelites.
Rabbi Eliezer offered a different reading: the verse speaks of a convert who backslid — someone who had joined the Jewish people but then reverted to his former practices. This person is still technically "your foe" because he has betrayed the covenant, yet the Torah still requires you to return his lost property.
Rabbi Yitzchak proposed the simplest reading: the verse speaks of an ordinary Israelite with whom you have quarreled. Why is he called "your foe"? Because he hit your son, or argued with you, or wronged you in some way. He becomes "your foe" temporarily, for that moment. And even then — even when you are angry at him — the Torah requires you to help him recover his lost animal.
Rabbi Yitzchak's reading is the most personally demanding. It is easy to help a stranger. It is harder to help someone who has personally wronged you. The Torah commands exactly that.