In Greek mythology, Cyllen (Ancient Greek: Κυλλήν) was an Arcadian, a son of Elatus, from whom Mount Cyllene in Arcadia was believed to have received its name. He is known almost entirely from the geographer Pausanias, who lists him among the five sons of Elatus and credits him with giving the mountain its name and the god Hermes the cult-surname "Cyllenian." No marriage, children, or deeds are attested for him; he survives essentially as the eponymous hero of the peak on whose summit Hermes was said to have been born.
Adapted from Wikipedia, "Elatus" · CC BY-SA 4.0
I.Family
Cyllen was an Arcadian prince, reckoned a son of King Elatus. According to Pausanias, Elatus was a son of Arcas (by Leaneira, Meganeira, Chrysopeleia or the nymph Erato, depending on the account) and the brother of Apheidas and Azan, the three among whom Arcas divided his kingdom.
By his wife Laodice, daughter of Cinyras, Elatus became the father of five sons: Aepytus, Pereus, Cyllen, Ischys, and Stymphalus. Cyllen is thus the brother of these four Elatids, several of whom are themselves eponymous or dynastic figures of Arcadia (Stymphalus giving his name to the town and lake, Aepytus serving as an early Arcadian regent, and Pereus becoming grandfather, through his daughter Neaera, of later heroes). No wife or children of Cyllen himself are recorded.
II.Mythology
When Arcas portioned out the land of Arcadia, Elatus was allotted the region around a mountain that until then had borne no name. Pausanias relates that it was Elatus's son Cyllen who afterwards gave the mountain its name, so that the highest peak in Arcadia came to be called Mount Cyllene.
The same eponym extended to a god: the temple of Hermes that stood (in Pausanias's day already ruined) on the summit of Cyllene was dedicated to "Cyllenian Hermes," and the mountain was famed as the birthplace of the god. Pausanias states plainly that Cyllen, son of Elatus, gave the mountain its name and the god his surname — the figure's entire mythological significance lying in this act of naming rather than in any narrated deeds.
III.Sources and attestation
Cyllen is an unusually thinly attested figure. The geographer Pausanias is effectively the sole ancient witness: he names Cyllen among Elatus's five sons in his survey of the Arcadian royal house (Description of Greece 8.4.4) and credits him with the naming of Mount Cyllene and the surname of Cyllenian Hermes (8.17.1).
Other early genealogies do not corroborate him. The mythographer Apollodorus, listing Elatus's offspring, names only Stymphalus and Pereus and makes no mention of Cyllen. Because Pausanias supplies only the patronymic and the eponym role, later reference works (such as the standard nineteenth-century dictionaries of classical biography that underlie modern encyclopedic summaries) record little beyond that Cyllen was a son of Elatus from whom the mountain took its name.
IV.Distinction from similar names
Cyllen the eponymous hero should not be confused with the closely related but distinct uses of the name in Greek myth. The recurring epithet "Cyllenian" or "Cyllenius" (e.g. in Homer's Odyssey and in Ovid) is a title of the god Hermes meaning "of Cyllene," not a reference to the hero Cyllen.
Likewise distinct are an "Otus of Cyllene," a Greek warrior mentioned in the Iliad and named for the mountain rather than the hero, and a Dactyl called Cyllenus in Apollonius's Argonautica. There is also a separate figure Cyllene, an Arcadian oread (mountain nymph) who nursed the infant Hermes and to whom the mountain's name is sometimes ascribed instead; she is a different, feminine figure from the male eponym Cyllen, son of Elatus.