Cercyon ("boar's tail") in Greek mythology is

a son of Poseidon by a daughter of Amphictyon, and accordingly a half-brother of Triptolemus, others call him a son of Hephaestus. (Hygin. Fab. 38.) He came from Arcadia, and dwelt at Eleusis in Attica. (Plut. Thes. 11; Ov. Met. vii. 439.)

But Choerilus, an Athenian, who wrote a play called Alope, says that Cercyon and Triptolemus were brothers, that their mother was the daughter of Amphictyon, while the father of Triptolemus was Rarus, of Cercyon, Poseidon. Paus. 1.14.3

Alope the daughter of Cercyon was with Poseidon the mother of Hippothoon.

Cercyon had his daughter buried alive but Poseidon turned her into the spring, Alope, near Eleusis.

Cercyon was the King of Eleusis, and a very strong man. He stood on the roads around Eleusis and challenged passers-by to a wrestling match. The loser (always the passer-by) was murdered, though Cercyon promised his kingdom to anyone who won. He was eventually beaten and killed by Theseus, who took the kingdom of Eleusis.

After the graves of the Argives is the tomb of Alope, who, legend says, being mother of Hippothoon by Poseidon was on this spot put to death by her father Cercyon. He is said to have treated strangers wickedly, especially in wrestling with them against their will. So even to my day this place is called the Wrestling Ground of Cercyon, being a little way from the grave of Alope. Cercyon is said to have killed all those who tried a bout with him except Theseus, who out matched him mainly by his skill. For Theseus was the first to discover the art of wrestling, and through him afterwards was established the teaching of the art. Before him men used in wrestling only size and strength of body. Pausanias 1.39.3


according to Apollodorus Cercyon killed by Theseus was a son of Branchus and Argiope

Fifth, in Eleusis he slew Cercyon, son of Branchus and a nymph Argiope. This Cercyon compelled passers-by to wrestle, and in wrestling killed them. But Theseus lifted him up on high and dashed him to the ground. Apollodorus

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Dictionary of Greek Mythology