Leon Trotsky adopted his famous pseudonym in 1902 when he was exiled to Siberia. In order to escape Tsarist authorities, he needed a name to put on a false passport. He reportedly chose the surname Trotsky after a jailer who worked at the prison in Odessa where he had previously been held.  "Leon" is simply the Westernized romanization of his birth first name, Lev. (Picture from 1918)