Political parties that are labeled as “extremists” by the western political scientists are the Communist Party of Soviet Union (it was called the Bolshevik Party) and the National-Socialist Party (Nazi party).
These two political parties did not appear suddenly from nothing, or their name, or their doctrine.
In their final form, the left wing party called Communist Party is that political party which has as a main teaching the Marxist-Leninist doctrine.
The right wing political party called National-Socialist Party has as a main teaching the Mein Kampf book by Adolf Hitler. The doctrine promoted by this book considers Aryan race superior over the other races.