It's a bizarre order, for animals to join humans in fasting and mourning sin. But the book of Jonah reveals that the King of Nineveh was so penitent about the sins of his people that he didn't want to leave anything to chance.
Jonah 3:7-8 reads, "And he caused it to be proclaimed and published throughout Nineveh by the decree of the king and his nobles, saying, Let neither man nor beast, herd nor flock, taste anything; do not let them eat, or drink water. But let man and beast be covered with sackcloth, and cry mightily to God; yes, let every one turn from his evil way and from the violence that is in his hands."
The salvation of Nineveh's people was absolutely incredible. The Bible describes the city as having a population of one hundred and twenty thousand people. It was so vast that it took three days just to walk across it. Yet in a short amount of time, Jonah's God-given message sparked a revival across this "exceedingly great city."
And ironically, the message of repentance came from a preacher who seemed very unwilling to give it. Not only did Jonah run the opposite direction of Nineveh when God told him to go there, but after one of the greatest spiritual revivals in history, Jonah complained to God that His message was so effective.
It goes to show you that we are but feeble vessels tasked with spreading the good news. It is only God who can bring sinners to true repentance.