There's a second plate of that metope that shows the full figures of both Perseus and Medusa and includes the winged horse Pegasus that was born of Medusa's blood.
the architectural element between two triglyphs in a doric frieze
الواصلة
the ar·chi·tec·tur·al el·e·ment be·tween two triglyphs in a Dor·ic frieze