Question: What son of Zeus in Greek mythology was said to have fatheres an Argonaut seer?

Context paragraphs:
Porthaon: In Greek mythology, Porthaon (Greek: Πορθάων, genitive Πορθάονος ), sometimes referred to as Parthaon or Portheus (seems related to the verb "portheō" and "perthō", "destroy'), was the king of Calydon, husband of Euryte and father of Oeneus, Agrius, Alcathous, Melas, Leucopeus and Sterope, also of the Argonaut Laocoön by an unnamed female servant, or by Euryte too.  He was the son of Agenor (by Epicaste) who was the son of Pleuron who was the son of Aetolus who was the son of Endymion.  He was the brother of Demonice (also known as Demodice).

Leda and the Swan: Leda and the Swan is a story and subject in art from Greek mythology in which the god Zeus, in the form of a swan, seduces Leda.  According to later Greek mythology, Leda bore Helen and Polydeuces, children of Zeus, while at the same time bearing Castor and Clytemnestra, children of her husband Tyndareus, the King of Sparta.  In the W. B. Yeats version, it is subtly suggested that Clytemnestra, although being the daughter of Tyndareus, has somehow been traumatized by what the swan has done to her mother (see below).  According to many versions of the story, Zeus took the form of a swan and seduced Leda on the same night she slept with her husband King Tyndareus.  In some versions, she laid two eggs from which the children hatched.  In other versions, Helen is a daughter of Nemesis, the goddess who personified the disaster that awaited those suffering from the pride of Hubris.

Rhacius: In Greek mythology, Rhacius (Greek: Ῥάκιος ) was the son of Lebes, and the leader of the first Greeks to settle in Caria, and became King of Caria.  His court was located at Colophon in Ionia.  With his wife Manto, daughter of the seer Teiresias, he was the father of Mopsus, a renowned seer.

Carnus: In Greek mythology, Carnus (also spelled Carneus and Carneius) was a seer from Acarnania.  According to the poet Praxilla, he was a son of Zeus and Europa.  He was reared by Leto and Apollo, and is also known to have been a lover of Apollo.

Anaxagoras (mythology): In Greek mythology, Anaxagoras (Greek: Αναξαγόρας ) was a king of Argos and son of either Megapenthes or his son Argeus.  The prince, Anaxagoras' son, suffered from a strange malady and the king offered a reward for anybody that could heal him.  Melampus, a local seer, killed an ox and talked to the vultures that came to eat the corpse.  They said that the last time they had had such a feast was when the king made a sacrifice.  They told Melampus that the prince had been frightened of the big, bloody knife and the king tossed it aside to calm the child.  It had hit a tree and injured a hamadryad, who cursed the prince with the sickness.  The hamadryad told Melampus that the boy would be healed if the knife was taken out of the trunk of the tree and boiled, then the rusty water that resulted drunk by the prince.  Melampus followed her directions and demanded two thirds of the kingdom for himself, and one third for his brother, Bias.  The king ultimately agreed.  When the women of Argos were driven mad by Dionysus, in the reign of Anaxagoras, Melampus was brought in to cure them, but demanded a third of the kingdom as payment.  The king refused, but the women became wilder than ever, and he was forced to seek out Melampus again, who this time demanded both a third for himself and another third for his brother Bias.  Sometimes, this story is told not of Anaxagoras, but of his grandfather, Proetus.  Anaxagoras was succeeded by his son Alector.  His house lasted longer than those of Bias and Melampus, and eventually the kingdom was reunited under its last member, Cyanippus.

Apollo: Apollo (Attic, Ionic, and Homeric Greek: Ἀπόλλων , "Apollōn" ( Ἀπόλλωνος ); Doric: Ἀπέλλων , "Apellōn"; Arcadocypriot: Ἀπείλων , "Apeilōn"; Aeolic: Ἄπλουν , "Aploun"; Latin: "Apollō" ) is one of the most important and complex of the Olympian deities in classical Greek and Roman religion and Greek and Roman mythology.  The ideal of the "kouros" (a beardless, athletic youth), Apollo has been variously recognized as a god of music, truth and prophecy, healing, the sun and light, plague, poetry, and more.  Apollo is the son of Zeus and Leto, and has a twin sister, the chaste huntress Artemis.  Apollo is known in Greek-influenced Etruscan mythology as "Apulu".

Phthonus: In Greek mythology, Phthonus (Greek: Φθόνος , "Phthónos") was the personification of jealousy and envy, especially in matters of love.  He was said to be the counterpart of Nemesis.  The god had a great many wives but killed each one eventually, on suspicions of adultery.  However, his power influenced not only him and mortals, but also the gods as well (most famously Hera).  According to some scholars, he was the son of Nyx and/or Dionysus.  But based on other documents, he was implied to be the son of Aphrodite.  In these, it was he that directed Hera's attention to Zeus's many affairs, including one of his most famous, that with the Theban princess Semele.  He also put the thought into Hera's mind to trick Semele into convincing Zeus into showing Semele his full glory, which killed her.

Echo (mythology): In Greek mythology, Echo ( ; Greek: Ἠχώ , "Ēkhō", "echo", from ἦχος ("ēchos"), "sound") was an Oread who resided on Mount Cithaeron.  Zeus loved consorting with beautiful nymphs and often visited them on Earth.  Eventually, Zeus's wife, Hera, became suspicious, and came from Mt. Olympus in an attempt to catch Zeus with the nymphs.  Echo, by trying to protect Zeus, endured Hera's wrath, and Hera made her only able to speak the last few words spoken to her.  So when Echo met Narcissus and fell in love with him, she was unable to tell him how she felt and was forced to watch him as he fell in love with himself.

Lycaon (Arcadia): In Greek mythology, Lycaon (/laɪˈkeɪɒn/; Greek: Λυκάων) was a king of Arcadia, son of Pelasgus and Meliboea, who, in the most popular version of the myth, tested Zeus by serving him the roasted flesh of Lycaon's own son Nyctimus, in order to see whether Zeus was truly omniscient.  In return for these gruesome deeds, Zeus transformed Lycaon and his offspring into the forms of a wolf; Nyctimus was restored to life.

Idmon: In Greek mythology, Idmon was an Argonaut seer.  Allegedly a son of Apollo, he had Abas (or Ampycus) as his mortal father.  His mother was Asteria, daughter of Coronus, or Cyrene, or else Antianeira, daughter of Pheres.  By Laothoe he had a son Thestor.  Idmon foresaw his own death in the Argonaut expedition, but joined anyway.  During the outbound voyage of "Argo", a boar killed him in the land of the Mariandyni, in Bithynia.

Supporting paragraphs: Idmon Apollo
Answer: Apollo