Apollodorus makes her a daughter of Thestius, and tells how Tyndareus, driven from Lacedaemon, fled to Thestius and married Leda; restored to his kingdom after Heracles slew Hippocoon, he reigned with her at Sparta. To Tyndareus she bore the daughters Timandra and Clytemnestra, married to Echemus and to Agamemnon, and Phylonoe, whom Artemis made immortal. Hyginus likewise calls her daughter of Thestius and wife of Tyndareus son of Oebalus.2Pseudo-Apollodorus · Hyginus — Bibliotheca 3.10.5–6; Fabulae 78

In Homer she appears in the underworld as Lede, wife of Tyndareus, who bore him two sons stout of heart, Castor the tamer of horses and the boxer Polydeuces; the life-giving earth covers them both, yet alive, for on Zeus's gift one day they live and the next are dead, and they hold honour as gods. In the Iliad Helen names Castor and Polydeuces her own brethren, whom the same mother bore. Apollonius too calls her the bride of Zeus, who sent her dearly loved twins from Sparta on the Argo.3Homer · Apollonius of Rhodes — Odyssey 11.298–304; Iliad 3.236–238; Argonautica 1.146–149

Apollodorus tells the signature myth: Zeus in the form of a swan consorted with Leda, and on the same night Tyndareus lay with her, so that she bore Pollux and Helen to Zeus and Castor and Clytemnestra to Tyndareus. Hyginus gives the same dual conception, the swan coupling with her by the river Eurotas. Ovid lists her among the loves of the god woven into Arachne's web, lying beneath the swan's white wings.4Pseudo-Apollodorus · Hyginus · Ovid — Bibliotheca 3.10.7; Fabulae 77; Metamorphoses 6.109

Against this stands the rival egg-tradition, which makes Leda only Helen's foster mother. Apollodorus reports that some say Helen was the daughter of Nemesis and Zeus: fleeing the god, Nemesis became a goose and Zeus a swan, and from their union she laid an egg that a shepherd found and gave to Leda, who reared the child as her own. Hyginus' Astronomica has Hermes carry the egg to Sparta and drop it in Leda's lap, leaving the matter undecided, and Pausanias records the relief of Pheidias at Rhamnus that showed Helen led to Nemesis by Leda.5Pseudo-Apollodorus · Hyginus · Pausanias — Bibliotheca 3.10.7; Astronomica 2.8.2; Description of Greece 1.33.7–8