Who was the pretty greek maiden who Zeus turned into a swan?

Zeus did not turn anyone into a swan except himself. To seduce Leda, Queen of Sparta he turned himself into a swan and laid with her. Some say she laid an egg from which were hatched the DiOSkouroi twins, Kastor and Polydeukes, both sons of Zeus. Others that she laid two eggs, each of which contained one child of the god--Polydeukes in one and Helene in the other--and one child sired by her mortal husband--namely, Kastor and Klytaimnestra. Yet others relate that the second egg, containing just Helene, was delivered to Leda by the goddess Nemesis who had lain with Zeus in the guise of a goose.