Lycaon was the son of Pelasgus and either the Oceanid Meliboea or Deianira, daughter of an elder Lycaon. His wife was called Cyllene, an Oread nymph who gave her name to Mount Cyllene, though sometimes she was regarded as his mother instead. Other accounts name his wife as Nonacris, after whom an Arcadian town may have been named.

Lycaon had at least three daughters: Callisto, Dia, and Psophis.

According to the Bibliotheca (Pseudo-Apollodorus), Lycaon had fifty sons. An alternate list is given by Pausanias, whose account states that almost each of them founded a city in Arcadia and became its eponym.

The sons include Nyctimus, who succeeded Lycaon as king, alongside such figures as Maenalus, Mantineus, and Orchomenus; the various ancient lists differ in the names they record, with most of the sons credited as founders and eponyms of Arcadian towns.

There are several versions of the Lycaon myth, already reported by Hesiod and told by several authors. The most popular is the one given by Ovid in the first book of his Metamorphoses.

In the Bibliotheca version, Lycaon had sired fifty sons with many wives, described as nefarious and carefree. Zeus visited in the form of a peasant, and the sons mixed the entrails of a child into the god's meal. The enraged Zeus threw the meal over the table (explaining the city name Trapezus, from "table") and killed Lycaon and his sons with lightning; only Nyctimus survived, through the intervention of Gaia. John Tzetzes gives two similar accounts agreeing with Apollodorus, one attributing the idea to Maenalus and another making Nyctimus the victim.

In Pausanias's version, Lycaon was instantly transformed into a wolf after sacrificing a child on Zeus's altar and sprinkling its blood upon it. In Lycophron's version, all were transformed into wolves for devouring Nyctimus, and these characteristics were extended to all Arcadians. The Fabulae and De astronomia versions resemble Pseudo-Apollodorus but describe the victim as Arcas, son of Jupiter and Callisto (Lycaon's daughter); restored to life, Arcas became a hunter, while his mother was turned into a bear, both ultimately becoming the constellations Arctophylax and Ursa Major.

In the version of Nicolaus Damascenus, Lycaon's sons were nefarious and, to test Zeus, mixed a boy's flesh into the sacrifices, whereupon all present during the murder were killed by lightning. In Ovid's version, Lycaon mistrusted the divine signs Zeus showed on arriving in Arcadia and, to test whether his guest was truly a god, served him the flesh of a prisoner, partly cooked and partly roasted; when Lycaon then attempted to murder the sleeping god, Zeus brought down the roof and transformed the fleeing king into a wolf.

The Suda relates that Lycaon diligently guarded the laws established by his father and, to keep his subjects lawful, claimed that Zeus frequented his home in mortal guise; when a sacrifice was being prepared, the people killed one of his fifty sons and mixed the body with the sacrificial meat to verify his claim, and all were killed by lightning. According to Eratosthenes, Lycaon butchered his grandson Arcas, whom Zeus reassembled and placed among the constellations, while Lycaon's house was struck by a thunderbolt.

In some versions the victim is Lycaon's grandson or an unidentified child, boy, or prisoner, and some variants state that it was his sons, rather than Lycaon himself, who decided to test Zeus.