Actually, a whipper-in is a who. When hounds are hunted in a pack, the whipper-in is the huntsman's assistant, so called because he (or she) has the job of riding around the hounds to keep them from wandering off. He or she also has to track down any hounds that do escape, and help stop the hounds if the pack needs to be called off. With live quarry, he also has to keep an eye out for the prey.
A hunt may have one or two whippers-in working on any given day.
Image source: Henrik Jessen via Wikimedia Commons.