The structure indicated is the anconeus muscle of the forearm.
The anconeus muscle is one of the muscles of the posterior compartment of the forearm. The posterior compartment consists of a superficial layer of muscles comprised of 7 muscles. The deep layer is comprised of 5 muscles.
Origin: lateral epicondyle of humerus
Insertion: olecranon, proximal surface of ulna
Innervation: Radial nerve
Action: assists elbow extension, stabilises the elbow joint, abducts ulna during pronation
Learn about the muscles of the forearm in this tutorial.