Whymper acts as "an intermediary between Animal Farm and the outside world" (6.7). He's the first human the pigs permit contact with after the Rebellion, constantly showing up in minor role to pass on communications from Napoleon to the other humans.
Whymper is "a sly-looking man with side whiskers, a solicitor [lawyer] in a very small way of business, but sharp enough to have realized earlier than anyone else that Animal Farm would need a broker and that the commissions would be worth having" (6.9).
In other words, he's in it for the money. And that makes him easy. As the harvests begin to shrink on Animal Farm, Napoleon uses Whymper to spread rumors that everything is fine, just fine. And then Whymper conveniently hears nothing when Napoleon squashes out the Hens' Rebellion by starving several of the hens.
Can you feel the burn? We can. Whymper is an allusion to all the Westerners who catered to Soviet interests and helped spread the Soviet myth for personal profit—or just because they were gullible. If you want a specific double, try the intellectual Lincoln Steffens. After visiting the Soviet Union in the early 1920s, Steffens said, "I've seen the future, and it works."
Well, says Orwell, look harder.