Animals and Grace
The book of Jonah is a polemic against Israel’s hatred of the Assyrians.
Every creature in this story is more righteous than Jonah, extending all the way to the plants, animals, and weather.
Everything obeys the call of God instantly, except the one person who God appointed to be a light to the nations.
Jonah is chosen for the most honorable role in the story, but he rejects it, choosing instead to live the life of an exile (eventually living in a booth east of Nineveh!).
His life had been spared in spite of his disobedience to the direct command of God, but he still lusts for an entire nation of people who hardly know right from wrong to go to their destruction.
Jonah demands that death take him so he doesn't have to witness the mercy of God on others.
So those animals who are fasting and repenting are proving they are more human than Jonah, who is becoming the actual beast of the story.
My hunch is that the wickedness of the Assyrians caused the Israelites to view them as less than human - like beasts. (Elsewhere, wicked people are described as "beasts" (בְּהֵמָה) in Psalm 73:22, Micah 5:8, etc.) For example, Nahum 2 describes Nineveh as a den of lions.
And while Assyria precedes the empires described as different beasts in Daniel 7, they certainly fit that same mold. So even if Jonah (who is representing Israel in the polemic) wanted to dehumanize the Ninevites as "beasts" (בְּהֵמָה) to justify their destruction, those "beasts" who repented became more human than Jonah.
This might be why Jonah 4 ends with God's love for both the Assyrian people and the many "beasts" of Nineveh. God takes no pleasure in the death of the wicked, but He desires to transform them through repentance (Eze. 33:11). Jonah (Israel) has become the antagonist who longs for death to overtake Nineveh and even his own life.
This pattern repeats in the NT as many Jews are rejecting salvation in their Messiah, so it is offered to the Gentiles in hopes of provoking the Jews to jealousy (Rom. 11:11). But they reject their role of being a light to those nations, and would rather persecute anyone who finds repentance in Christ. So, just as God used Assyria to judge Israel, Rome would then be used to judge Judah.
