Greek mythology features four women named Pelopia.
One Pelopia was one of the six daughters of Niobe, slain by Artemis in punishment for Niobe's hubristic vanity, when Niobe asserted that she was more splendid than Leto, having borne six daughters and six sons, rather than only two children (Apollo and Artemis).
The second Pelopia was, by Ares, the mother of Kyknos, the bandit prince at Pagasia near the Thessalian coast, who was turned into a swan after losing a duel against Herakles.
Another Pelopia was a daughter of Pelias and either Phylomache (daughter of Amphion) or Anaxibia. In the Argonautica she gives her brother Acastus a double-folded mantle to take with him on the Argo during his travels with Jason. Along with her sisters, she killed her father, believing due to a trick by Medea that this would actually save her elderly father's life from old age and illness.
The fourth Pelopia, also called Mnesiphae, was the daughter of Thyestes, raped by him in an effort to father upon her a son who would (according to oracular prophecy) kill Atreus, her uncle and the main competitor for the throne of Mycenae. This event was preceded by numerous other criminal actions between Thyestes and his brother Atreus: Thyestes had had an affair with Aerope, Atreus' wife; Atreus then killed Thyestes' sons and fed them to Thyestes at a banquet. At the occasion of the rape, Pelopia pocketed the dagger of her masked assailant, with the intent to use it later to identify him to the court of king Thesprotus of Sicyon, with whom she was staying at the time.
Atreus visited Thesprotus, trying to find Thyestes, and met Pelopia, not recognizing his own niece, but mistaking her for Theosprotus' own daughter. Atreus requested to marry her, which Thesprotus allowed, preferring to keep it secret that the girl had been violated, which could impair her chance for a good marriage. Some months after the wedding, Pelopia bore Aegisthus, Thyestes' son, and she exposed him to the elements, that he might die. A she-goat found the infant and suckled him, until a local shepherd likewise found the babe and brought him to Atreus, who raised him as his own son.
Once Aegisthus became an adult, Pelopia gifted him the dagger she had stolen from her rapist, seeing no other purpose for retaining it so long after the event, and not wanting to explain its provenance. At Delphi, Agamemnon and Menelaus captured Thyestes and brought him before Atreus, who directed Aegisthus to slay Thyestes. Thyestes saw Aegisthus' dagger, recognizing it as his own, and asked the young man where he had obtained it; Pelopia was brought forward to tell of the dagger's origin, as she had given it to him. Upon realizing that Thyestes was her rapist, Pelopia took the dagger and killed herself with it. Then Aegisthus slew Atreus, his adoptive father and biological uncle/granduncle, returning Thyestes to the throne of Mycenae.
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