From the New York Times: An Altar Beyond Olympus for a Deity Predating Zeus

Before Zeus hurled his first thunderbolt from Olympus, the pre-Greek people occupying the land presumably paid homage and
offered sacrifices to their own gods and goddesses, whose nature and identities are unknown to scholars today.

But archaeologists say they have now found the ashes, bones and other evidence of animal sacrifices to some pre-Zeus deity on the summit of Mount Lykaion, in the region of Greece known as Arcadia. The remains were uncovered last summer at an altar later devoted to Zeus. [continue]