Mr. Pilkington owns Foxwood, a neighboring farm. And—he doesn't sound so bad, really. The narrator tells us that he's "an easy-going gentleman farmer who spent most of his time in fishing or hunting according to the season" (4.2). Well, easy-going except when it comes to his other neighbor, Mr. Frederick: "These two disliked each other so much that it was difficult for them to come to any agreement, even in defence of their own interests" (4.2).
Do you get it?
Okay, we'll tell you: Mr. Pilkington is a symbol for the West (the U.S. and U.K.) and his quarrelsome neighbor is Germany. Both Pilkington and Frederick are worried that revolutionary spirit might spread to their own animals: Pilkington calls it a "terrible wickedness" to keep his animals in place (4.3).
Things get tricky when Napoleon re-opens business relations with humans. He sometimes deals with Pilkington and sometimes with Frederick, but never with both simultaneously. For a while, Napoleon's relations with Pilkington are "almost friendly," and Napoleon seems to hate Frederick as much as Pilkington does (8.7). At the last minute, however, Napoleon does a switcheroo and sells some promised timber to Frederick. The next thing you know, the animals on the farm are chanting "Death to Pilkington" (8.11).
But then Frederick cheats Napoleon. The pigs send over some pigeons to Pilkington, hoping to patch up their relations. But it can't be done. The pigeons return with a simple message: "Serves you right" (8.16). The relations between Animal Farm and Foxwood appear to be severed.
What's up with all this backdoor dealing? The entire episode alludes to Stalin's non-aggression pact with Hitler in 1939. See, Hitler and Stalin had been enemies for years. Hitler was a fascist, and communists in the 1930s were basically anti-fascist by definition. (Quick Brain Snack: fascists believed in unquestioning loyalty to a strong military state headed by an authoritarian leader.)
In the 1930s, Stalin had almost signed on to an anti-German alliance with Britain and France, but the alliance fell apart at the last minute. So, Nazi Germany and Stalin's Russia signed a non-aggression pact in 1929, left-leaning idealists (like one Mr. George Orwell) were super bummed out. They still believed in the supposed socialist ideals of the Soviet Union, and the non-aggression pact make Stalin look like just another tyrant.
Only when Hitler broke the pact by launching a massive surprise invasion of Russia in June 1941 did Stalin suddenly remember that he was supposed to be an anti-fascist—and at that point, Stalin and the West became buddy-buddy again.
At the end of the novel, Pilkington is having a fancy dinner at Napoleon's farmhouse. (We're guessing pork chops are not on the menu.) The scene probably represents the Tehran Conference, a 1943 meeting between the leaders of the United Kingdom (Prime Minister Winston Churchill), the United States (President Franklin D. Roosevelt), and the Soviet Union (uh, Stalin, in case you haven't been paying attention). These dudes agreed how to handle the rest of the war and started planning what to do with Europe after the war—like give most of Eastern Europe to the Soviet Union. (Hungry for more details on this fascinating Tehran Conference? See "Symbols, Imagery, Allegory.")
Since Animal Farm ends on this note, we're feeling… a little uneasy. Pilkington toasts Napoleon, and jokes, "If you have your lower animals to contend with, we have our lower classes!" (10.24). LOL, right? And then the book ends with Pilkington and Napoleon scowling at each other when they "both played an ace simultaneously."
And it's the same ace.
In other words, one of them has been cheating (10.31). The alliance is artificial, tense, and doomed. Pretty prophetic, Mr. Orwell.