- The butterfly genus Cycnus is now synonymized with Panthiades.
Son of Ares 
Cycnus, son of Ares by Pelopia or Pyrene, was a bloodthirsty and cruel man who dwelt either in Pagasae, Thessaly or by the river Echedorus in Macedonia, and killed all of his guests until he was slain by Heracles. According to Pausanias, one of the men murdered by him was Lycus of Thrace.
Pseudo-Apollodorus wrote of Cycnus the Thessalian, the son of Pelopia, and Cycnus the Macedonian, the son of Pyrene, as two distinct encounters of Heracles, mentioning them separately. The Thessalian Cycnus, he relates, challenged Heracles to single combat and was killed by him; the same is recounted by Diodorus. The Macedonian Cycnus, according to the Bibliotheca, also challenged Heracles to single combat; Ares attempted to avenge his son's death, but a thunderbolt was hurled by Zeus between the combatants, causing them to part.
In the poem Shield of Heracles, Heracles and Iolaus encountered Cycnus and Ares on the way to Trachis, each pair riding a chariot; it is mentioned that Heracles was directing to the court of King Ceyx, to whose daughter Themistonoe Cycnus was married. Apollo, whose Pagasaean sanctuary was next to the place where the characters meet, is said to have stirred Heracles up against Cycnus. Athena appeared, telling Heracles that Zeus too had empowered him to defeat Cycnus, and instructed him not to touch his body or take his armor as spoils, but to hit Ares with a spear in an unprotected part of body if he should attack Heracles to avenge Сycnus' death. Heracles and Сycnus then clashed in single combat and Heracles hit Сycnus in the neck with a spear, killing him. Ares was just about to strike a hit at Heracles when Athena blocked his hit, saying that it was not ordained that he should kill the hero. Heracles took this opportunity to wound Ares in the thigh and beat him to the ground, which caused his sons Phobos and Deimos to rescue him and take him back to Mount Olympus. Сycnus was buried by his father-in-law Сeyx and people of the neighboring cities, but his tomb was destroyed during a flood by the will of Apollo.
King of Kolonai 
As king of the town of Kolonai in the southern Troad, Cycnus was the son of Poseidon by Calyce (daughter of Hecaton), Harpale, or by Scamandrodice. According to John Tzetzes, his mother Scamandrodice abandoned him on the seashore, but he was rescued by fishermen who named him Cycnus "swan" because they saw a swan flying over him. In another account, he was said to have had womanly white skin and fair hair, which was why he received his name that meant "swan".
Cycnus married first Procleia, daughter of Laomedon (King of Troy) or of Laomedon's son Clytius. Cycnus and Procleia had two children, named Tenes and Hemithea, although Tenes claimed the god Apollo as his father. On Procleia's death, Cycnus married Philonome, daughter of Tragasus (Cragasus), also known as Polyboea or Scamandria. Philonome fell in love with her handsome stepson, Tenes. Tenes rejected Philonome's advances, whereupon Philonome falsely accused Tenes before her husband of having ravished her. Cycnus ordered to place both his children in a chest and throw it into the sea. However, Cycnus discovered the truth and had Philonome buried alive. When he found that his children had survived and were reigning at Tenedos, he sailed there intending to reconcile with them, but Tenes cut the anchor rope of his ship.
Dictys Cretensis mentions three more children of Cycnus: two sons, Cobis and Corianus, and a daughter Glauce.
Cycnus later supported the Trojans in the Trojan War, and fought valiantly, killing one thousand opponents according to Ovid. According to some accounts he killed the Greek hero Protesilaus, but according to others, Cycnus attacked the Greek camp when the funeral of Protesilaus was underway. It was said that Cycnus, being the son of Poseidon, was invulnerable to spear and sword attack. When Achilles confronted Cycnus he could not kill him via conventional weaponry so he crushed and suffocated him. After his death, Cycnus was changed into a swan. Later, the Greek army invaded Cycnus' kingdom, but the people of Colonae implored them to spare the city. The Greek leaders agreed, on condition that Cobis, Corianus and Glauce be handed over to them, and made a truce with the citizens.
Friend of Phaethon 
Cycnus, king of Liguria, was the son of Sthenelus and a good friend or lover of Phaethon; according to Ovid, he was a distant relative of Phaethon on mother's side. After Phaethon died, he sat by the river Eridanos mourning his death. The gods turned him into a swan to relieve him of his pity. Even then he retained memories of Phaethon's death, and would avoid the sun's heat because of that. According to Virgil, Cycnus lamented Phaethon's death till he grew old, so his gray hair became gray feathers upon his transformation. Pausanias mentions Cycnus, king of the Ligyes (Ligurians), as a renowned musician who after his death was changed into a swan by Apollo. Servius also writes of Cycnus as a musician and a lover of Phaethon, and states that he was changed into a swan and later was placed among the stars by Apollo (that is, as the constellation Cygnus). Cycnus' talent of a musician motivates his association with the concept of the swan song suggested in Hyginus' account.
Servius also mentions that Сycnus had a son named Cupavo.
Son of Apollo 
Yet another Cycnus was a son of Apollo by Hyrie or Thyrie, daughter of Amphinomus; the story concerning him is recorded by Antoninus Liberalis. According to said author, he dwelt in the country between Pleuron and Calydon and dedicated most of his time to hunting. He was good-looking but arrogant and disrespectful towards numerous other youths who became enamoured of him and sought his attention. His attitude eventually made all of those youths desert him; only one of them, Phylius by name, loved him deeply enough to stay by his side nevertheless. Cycnus was still unmoved by Phylius' devotion and challenged him to three impossible tasks, hoping to get rid of him. The first task was to kill a lion that was threatening the neighborhood without use of any weapons. Phylius consumed a lot of food and wine and then vomited it back up on the spot where the lion would usually show up; the beast ate the products and became intoxicated with wine, whereupon Phylius strangled it with his own clothes. The second task was to catch two man-eating vultures of enormous size that were posing an equal threat to the neighborhood, again without use of any devices. While Phylius was contemplating a way to fulfill the task, he saw an eagle accidentally drop its prey, a dead hare, to the ground. Phylius then smeared himself with the hare's blood and lay still on the ground, pretending to be dead. When the vultures attacked him, he caught them by the feet and brought them to Cycnus.
Finally, Phylius had to bring a bull to the altar of Zeus with his own bare hands. Not being able to come up with a way to perform this last task, he prayed to Heracles for help. Then he saw two bulls fighting over a heifer and waited till in the course of the fight both fell to the ground and became helpless, which made it possible for him to grab one of the bulls by the legs and drag it to the altar. At this point, Heracles caused Phylius to no more obey the orders of Cycnus. When Cycnus found that, he felt disgraced and committed suicide by throwing himself into a lake called Conope; his mother Thyrie did the same. Apollo changed them both into swans. The lake became known as the Swan Lake because of that, and when Phylius died, he was buried near it.
Ovid also incorporates the story of Cycnus and Phylius in his Metamorphoses: in his version, Phylius performs the three tasks but refuses to deliver the tamed bull to Cycnus. The latter is scorned and throws himself off a cliff, but transforms into a swan as he is falling and flies away. His mother Hyrie, unaware of the transformation and thinking that he is dead, dissolves away in tears, thus changing into the lake Hyrie.
Other characters 
The name Cycnus may also refer to:
- Cycnus, son of King Eredion of Achaea, who, in one version, seduced Leda and made her mother of triplets: the Dioscuri and Helen. In all other sources, she had these children by Zeus who approached her in the shape of a swan (kyknos).
- Cycnus, a blunder for Guneus in the manuscript of Hyginus' Fab. 97 (list of the Achaean leaders against Troy).
- This scene has also been interpreted as a Gigantomachy. H. Shapiro, "Herakles and Kyknos", AJA, Vol. 88, No. 4 (October 1984), p. 524.
- Pausanias, Description of Greece, 1. 27. 6
- Pseudo-Apollodorus, Bibliotheca, 2. 7. 7. & 2. 5. 11
- Apollod. 2. 7. 7
- Diodorus Siculus, Library of History, 4. 37. 4
- Ps.-Apollod. 2. 5. 11; same version in Hyginus, Fabulae, 31
- Shield of Heracles, passim
- Euripides, Heracles, 390
- Eustathius on Homer, p. 254
- Strabo, Geography, 13. 1. 19
- Hyginus, Fabulae, 157; Scholia on Pindar, Olympian Ode 2. 147; Tzetzes on Lycophron 232
- Tzetzes on Lycophron, 232
- Scholia on Theocritus, Idyll 16, 49
- Scholia on Iliad, 1. 38
- Scholia on Ovid, Ibis, 463
- Apollodorus. The Library, Epitome of Book 4, 3.23-3.24.
- Conon, Narrations, 28
- Tzetzes on Lycophron, 232-233
- Pausanias, Description of Greece, 10. 14. 2 - 3
- Diodorus Siculus, Library of History, 5. 83. 4
- Dictys Cretensis, 2. 13
- Quintus Smyrnaeus, Fall of Troy, 4. 529
- Dictys Cretensis, 2. 12
- Ovid, Metamorphoses, 12. 64 - 145
- Ovid, Metamorphoses, 2, 367 sqq.
- Virgil, Aeneid, 10. 189 ff
- Pausanias, Description of Greece, 1. 30. 3
- Servius on Aeneid, 10. 189
- Hyginus, Fabulae, 154
- Antoninus Liberalis, Metamorphoses, 12
- Ovid, Metamorphoses, 7. 371 ff
- Pseudo-Apollodorus, Bibliotheca, Epitome of Book 4, 7. 27
- Tzetzes on Lycophron, 506
- Pseudo-Eratosthenes, Catasterismi, 25
- Hyginus, Poetical Astronomy, 2. 8