Also containing Pisa, this is, for ten points, what ancient Kingdom allegedly founded by Endymion?
In The Iliad, one King from this polity goes into a frenzy and kills Itymoneus and Agelaos after Paris errantly shoots Euenor, and* Nestor nostalgically recalls a war with this polity over grazing rights. A legendary set of twins from this polity were supposedly born conjoined from a silver egg after Poseidon seduced their mother, Queen Molione of Boupraison. The port city of Kyllene in this polity lay across from the island of Doulikhion, whose most famous King was Meges, while the throne of another of this polity's cities was seized after Myrtilos sabotaged the chariot of King Oinomaos. Foreknowledge of his death led the legendary founder of this polity to hold a chariot race among his sons for the throne, and this polity's royal line traces its roots back through Aethlios to Deucalion. Herakles twice invaded this Kingdom after being offended after his fifth labor by its King, Augeius, and after winning he established an altar to a legendary King of one of this polity's other cities, Pelops. Also containing Pisa, this is, for ten points, what ancient Kingdom allegedly founded by Endymion? 
In The Iliad, one King from this polity goes into a frenzy and kills Itymoneus and Agelaos after Paris errantly shoots Euenor, and* Nestor nostalgically recalls a war with this polity over grazing rights.
{Elis} or {Helia} (accept {Doulikhion} before *)