Snowball was one of the pigs on 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" (chapter 5, pg 43)
Snowball represents Trotsky because they were both co-leaders of a communist government, exiled from their own society while the other leader took total control, and sentenced to death for alleged treason
"Arrested at the age of 19 for writing and printing revolutionary literature and for helping organize a strike...sent to Siberia"
"Do you know who has...overthrown our windmill? Snowball!...I pronounce a death sentence upon Snowball" (chapter 6, pg 62)
This relates to the theme of how greed and power corrupts the government, because when Napoleon wanted all of the power of the farm, he saw Snowball as a threat to gain that power for himself, so he chased off Snowball with the dogs in order to take total control of the farm and used his abandonment to make Snowball look bad