Pausanias, 1.33.7

“The Greeks claim that Nemesis was Helen’s mother and that Leda nursed her and raised her.”

Ἑλένῃ Νέμεσιν μητέρα εἶναι λέγουσιν Ἕλληνες, Λήδαν δὲ μαστὸν ἐπισχεῖν αὐτῇ καὶ θρέψαι

Scholia to Lykophron 88

“Zeus made himself look like a swan and joined Nemesis near the river Ocean. From this union, she laid an egg which Leda received, warmed, and then bore Helen and the Dioscouri”

κύκνῳ ἀπεικασθεὶς ὁ Ζεὺς Νεμέσει τῇ ᾿Ωκεανοῦ συνῆλθεν, ἐξ ἧς γεννᾶται ᾠόν, ὅπερ λαβοῦσα ἡ Λήδα ἐθέρμαινε καὶ ἔτεκε τὴν ῾Ελένην καὶ τοὺς Διοσκούρους.

Scholia to Callimachus’s Hymns 3.232

“Ramnos is a deme in Attica where Zeus slept with Nemesis who then produced an egg which Leda found, warmed and which produced in turn the Dioscuri and Helen.”

<῾Ραμνουσίδι:> ῾Ραμνοῦς δῆμος ᾿Αττικῆς, ἔνθα τῇ Νεμέσει ὁ Ζεὺς συνεκαθεύδησεν, ἥτις ἔτεκεν ᾠόν, ὅπερ εὑροῦσα ἡ Λήδα ἐθέρμανε καὶ ἐξέβαλε τοὺς Διοσκούρους καὶ τὴν ῾Ελένην.

The fragmentary poem from the  epic cycle dubbed the Cypria was attributed to lesser known poets like Stasinus and Hegesias by ancient authors. Its name, however, comes from the fact that it was largely believed to be composed in Cyprus (or by a Cypriot poet traveling abroad).

The first fragment of the poem tends to be its most well-known since it places the Trojan War in a context of global discussion and echoes the Iliad in making this all part of Zeus’ plan. But the ninth fragment has some frightening details. First, it alleges that Helen is not the daughter of Zeus and Leda (of the swan scene) but instead is the offspring of Zeus and the unwilling goddess Nemesis. Second, it shows Zeus pursuing her all over the earth no matter what form she took.

Cypria, Fr. 9 Benarbé [fr 10. West 2013]

“After them [he?] bore a wonder to mortals, a third child Helen—
Fine-haired Nemesis gave birth to her after having sex
With Zeus, the king of the gods, under forceful compulsion.
For she was not willing to have sex with Kronos’ son
Father Zeus, since her mind rushed with shame and opposition [nemesis].
She fled over the earth and the dark, barren sea,
But Zeus pursued her—and he longed to catch her in his heart.
At one time along the waves of the much-resounding sea,
He broke through the water as she took the form of a fish—
At another he followed her through the river Ocean to the ends of the earth.
Again, across the much-nourishing land. She became all the terrible
Beasts, the many the land raises up, in trying to escape him.”

τοὺς δὲ μέτα τριτάτην ῾Ελένην τέκε θαῦμα βροτοῖσι·
τήν ποτε καλλίκομος Νέμεσις φιλότητι μιγεῖσα
Ζηνὶ θεῶν βασιλῆϊ τέκε κρατερῆς ὑπ’ ἀνάγκης·
φεῦγε γὰρ οὐδ’ ἔθελεν μιχθήμεναι ἐν φιλότητι
πατρὶ Διὶ Κρονίωνι· ἐτείρετο γὰρ φρένας αἰδοῖ
καὶ νεμέσει· κατὰ γῆν δὲ καὶ ἀτρύγετον μέλαν ὕδωρ
φεῦγε, Ζεὺς δ’ ἐδίωκε—λαβεῖν δ’ ἐλιλαίετο θυμῶι—
ἄλλοτε μὲν κατὰ κῦμα πολυφλοίσβοιο θαλάσσης
ἰχθύι εἰδομένην πόντον πολὺν ἐξοροθύνων,
ἄλλοτ’ ἀν’ ᾿Ωκεανὸν ποταμὸν καὶ πείρατα γαίης,
ἄλλοτ’ ἀν’ ἤπειρον πολυβώλακα· γίγνετο δ’ αἰνὰ
θηρί’, ὅσ’ ἤπειρος πολλὰ τρέφει, ὄφρα φύγοι νιν.

As West (2013, 81-83) points out, there is some motif transference going on here in the fragment. For one, in many testimonia Thetis is said to change shapes to elude Peleus. In addition, we know the popular account of Zeus changing into a swan [or goose] to seduce Leda. Finally, Nemesis—as a concept and less as a character—is often associated with Helen’s behavior. She receives “nemesis and shame” for her actions. Much of this may linger in the mythopoetic background when the leaders of the Trojans declare upon seeing her again in the Iliad “there’s no nemesis for the Trojans and Achaeans, that they suffered pain for so long for this kind of woman….” (οὐ νέμεσις Τρῶας καὶ ἐϋκνήμιδας ᾿Αχαιοὺς
τοιῇδ’ ἀμφὶ γυναικὶ πολὺν χρόνον ἄλγεα πάσχειν).

But other accounts have Zeus changing to match Nemesis as well. Apollodorus (3.10.7) attempts to harmonize the two accounts:

“Some allege that Helen is the daughter of Nemesis and Zeus and that when she was fleeing Zeus’ sexual advance she changed her shape into a goose and that Zeus matched her and approached her as a swan. She produced an egg from this intercourse—people say that some shepherd found this egg in a thicket, fetched it and gave it to Leda who placed it in a box where she guarded it. When, after some time, it hatched and produced Helen, Leda raised her as her own daughter.”

λέγουσι δὲ ἔνιοι Νεμέσεως ῾Ελένην εἶναι καὶ Διός. ταύτην γὰρ τὴν Διὸς φεύγουσαν συνουσίαν εἰς χῆνα τὴν μορφὴν μεταβαλεῖν, ὁμοιωθέντα δὲ καὶ Δία κύκνῳ συνελθεῖν· τὴν δὲ ᾠὸν ἐκ τῆς συνουσίας ἀποτεκεῖν, τοῦτο δὲ ἐν τοῖς ἄλσεσιν εὑρόντα τινὰ ποιμένα Λήδᾳ κομίσαντα δοῦναι, τὴν δὲ καταθεμένην εἰς λάρνακα φυλάσσειν, καὶ χρόνῳ καθήκοντι γεννηθεῖσαν ῾Ελένην ὡς ἐξ αὑτῆς θυγατέρα τρέφειν…