In his short novel Animal Farm, George Orwell used pigs to portray Trotsky and especially Stalin and Russia because the novel was allegorical and was published in 1945, right after the end of WWII. Russia and England had been allies in the war, and this novel was extremely critical of Stalin and Russia which would have embarrassed the British government. By using animals, Orwell can detail his animosity towards Stalin who had killed so many of his own people in the war, misused his power, had a secret pact with Hitler while fighting him publicly, and the Communist system which he saw as corrupt in its use of constant arrests and roaming armed police, without naming names. The use of animals also allows the reader to apply the criticisms of the system to any country instead of just one so that Orwell can emphasize the universality of his message. Pigs are noisy, smell bad, and love the mud which is not a flattering description when applied to a person. Thus, the" Animal Farm: A Fairy Story" the book was first titled with could raise the ugly picture of Stalin and Trotsky as being pigs in the mud rather than as noble leaders who had saved Britain from invasion.
It is because he wanted to add the artistic fuse and to also embarass both men and in a sense call how they behaved like animals.