In Animal Farm which animal voluntarily leaves the farm?

Snowball the pig leaves the farm, but he is forced out by napoleon (another pig) and his nine vicious dogs. I history, this relates to the exile of trotsky during the Russian Revolution, oonwhich the book is actually set.

This may be but Molly the horse left the farm and can be seen as the Vain, selfish people ofr Russia at the time of the revolution.