In ancient Greece, an agonist was a competitor in public games. In literature etc, someone involved in a conflict of some sort...ie often the chief character in a story. The word comes from the Greek 'agonia', meaning struggle.
Agonist is a kind of muscle that causes movement to occur. They create the normal range of movement in a joint by contracting.
An antagonist is a kind of muscle that acts in opposition to the movement generated by the agonist and is responsible for returning a limb to its initial position.
An organist is a different kind of muscle altogether!