Snowball was one of the pigs of the farm, but unlike the other pigs, he wasn't as manipulative as they were, and relied on fairness to win the other animals over "At the meetings Snowball often won the majority over by his brilliant speeches, but Napoleon was better at canvassing support for himself in between times" (pg 43)
Napoleon saw him as a threat to the pig's domination of the farm, so he had Snowball chased off the farm with the help of the dogs, this goes back to the theme of how greedy people with power corrupt the system and try to win by force
Snowball represents Trotsky because both were leaders of a communist government and were exiled from their own society and was sentenced for striking against them
"Arrested at the age of 19 for writing and printing revolutionary literature and for helping organize a strike...sent to Siberia"