What is "croups"

Usage examples of "croups".

Then, holding the tails, they slipped off the horses' croups and galloped on foot behind them, going as fast as the horses did.

Then they hauled themselves up the tails, leapt onto the croups and bounced forward to the saddles again, and rode standing on them, and then, incredibly, standing on their heads—the horses still going at stretch-out gallop.