In Orwell's Animal Farm, published in 1945, we learn early on that one of the tenets of the philosophy, which later became known as Animalism, was that pigs should not engage in trade.
Later, however, this principle is violated when Napoleon realizes that their farm cannot produce everything that they need. Accordingly, he decides that they should trade with neighboring human-run farms:
not, of course, for any commercial purpose, but simply in order to obtain certain materials which were urgently necessary.
Some of the animals recalled the resolution they had made against trade, but Squealer managed to convince them that no such resolution had ever been made.
Of course, the initiation of trade between the animals and the humans is one of the things that marks the corruption of their original principles. Thus, by the end of the novella, pigs and humans are openly interacting and it becomes difficult to tell the two apart:
Twelve voices were shouting in anger, and they were all alike. No question, now, what had happened to the faces of the pigs. The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.