Who does Snowball symbolize?

Snowball, in the allegory of Animal Farm, corresponds to Leon Trotsky.
As Trotsky had helped seed the ideology and command the army of the Russian Revolution, Snowball helps to spread the ideas of and bring about the revolution against the farmer.
Both held positions of power in the early days of the new regime. Both disagreed with their fellow leaders, which created animosity and rivalries. Snowball's differences in ideology were, of course, much more simple than Trotsky's differences. Orwell focused mainly on Snowball's desire to spread the revolution to other farms. Trotsky had a similar idea about the Communist revolution: that they should spend much more of their time and energy on spreading it to other nations.
Orwell seems to have sympathy for Trotsky, although he does offer some satirical jabs at him, such as in the failed committees Snowball keeps trying to organize. His heart is in the right place, but he seems pretty bad at leadership.
Snowball and Trotsky both were driven out by rivals who had gained more power. Stalin and Trotsky competed rather fervently for leadership of the nation after Lenin's death. Stalin, having organized his followers more effectively, defeated and exiled Trotsky and eventually had him killed. Napoleon has his dogs chase Snowball off the farm.