Marble votive relief representing Zeus Meilichios in the shape of a
snake, facing a group of worshipers (IV century B.C.), discovered in
Piraeus and now in the National Museums (Staatliche Museen) in Berlin,
Germany.
Zeus Meilichios (“Mild”, “Gracious” in the sense
of benevolent, easy to be entreated) was a particular form, abiding
in the Underworld, of the supreme god of the Greek pantheon, who was
generally associated with the heaven. Like many other mythological figures
related to the chthonian world, this form of Zeus could manifest himself
in the shape of a serpent. The serpent, associated with the world beyond
and with the dead, had a twofold aspect: both positive and negative,
benevolent and awful, like the subterranean Zeus himself.
[Source: http://library.artstor.org/library/]