Huh? Picus was a mythical Italian king, the knowledge of whose tradition the Greeks did not know until quite late. In fact, to my knowledge John of Nikiu, and early medieval writer, is the first Greek writer to mention Picus at all, and I believe the only one. I fail to see what connection he might have with Zeus, other than that Virgil maintained that he was the son of Saturn. But in the Roman estimation Saturn was a mortal man, an Italian king of great reputation, who only became (slightly) associated with the Greek Cronus later on. Picus was a decidedly local figure, the eponymous ancestor of the Piceni, with no relations in the tradition to the Greeks. John of Nikiu is the only text that preserves (provided he did not outright fabricate it, a not uncommon practice in his time) a tradition that Belus had anything to do with Zeus; all other traditions maintain that Poseidon and Agamede were the parents of Belus, Actor, and Dictys. John appears to be grasping at the similarity in the names of Rhea, Zeus' mother, and Rhea Silva, Romulus and Remus' mother, to whom Ovid says Picus (the woodpecker) appeared in a dream, without realizing that Rhea Silva's name is a poetic moniker for Ilia, and means "Rhea of the woods." The whole point of the moniker is to associate her with Hesiodic foundation stories.

These sorts of dodgy mythic foundation stories were common in Greek universal histories from the Hellenistic Period on, and John was imitating them in his piece. But John lived centuries after Christianity's dominance had been established in Egypt, even after the Muslims had taken over the area. It is highly unlikely he preserved any older pagan tradition than those we have in our classical texts. Indeed, his story mentions several characters that did not exist earlier. Hermes Trismegistus was a result of the syncretism of Greek and native Egyptian religions who appeared first in the Hellenistic Period and especially in late antiquity, when he was taken over by the Neoplatonists. John appears to be doing whatever he can to conflate Picus for whatever reason with as many foundational deities as he possibly can, even foundational deities whose cults did not exist until close to his own time. The connections between these traditions is therefore not even spurious, as John's link to Trismegistus must be a fabrication, or at best a tradition that only predated him by a century or two--hardly the stuff of Proto-Indo-European myth. Revisiting these late antique and early medieval mythographies was something of a habit in Harris' day, hot on the heels of The Golden Bough and its claims of far-reaching, unified global mythology.