The horse/donkey pairing is a poetic device; there's no intended technical distinction between the two. If you're familiar with satyrs in Roman mythology, it's sort of like a poetic comparison to them, but using terms familiar to the audience at the time.

The point of the passage is to call out Israel's spiritual lewdness and adultery in going after idols instead of worshipping the God who had made a covenant of faithfulness with them.