Alongside the familiar story of Zeus visiting Leda in the form of a swan, a less common but well attested tradition makes Nemesis herself the swan's true quarry, and the myth survives specifically because two very different kinds of source preserved it. Fragments of the lost epic Cypria, part of the material that once filled out the story of the Trojan War around Homer, describe Nemesis fleeing Zeus's advances by shifting from shape to shape, a fish, then a wild animal, before Zeus, transformed into a swan, catches and mates with her in that final form. The mythographer Apollodorus, writing centuries later in his Bibliotheca, preserves a fuller version in which Nemesis, having become a goose, lays an egg that a shepherd finds and brings to Leda, who keeps it until Helen hatches from it, so that Leda functions as a foster mother rather than a birth mother.
The two traditions were never reconciled into one official account, and ancient sources hold both the plain Leda and swan tale and this more elaborate Nemesis version without treating either as the corrected version of the other, an ambiguity the sources themselves seem untroubled by. What both agree on is a deeper point about Helen's parentage: making her the daughter of Nemesis, the goddess who punishes overreach, casts the Trojan War before it even begins as retribution built into Helen's very origin, a fate embedded in her birth rather than only in Paris's later choice.