There was a discussion recently on the Mythology Stack Exchange about whether Poseidon had been the first head of the Greek pantheon. It’s an interesting question….
You can argue this one in a number of ways. While the name Zeus is clearly Indo-European (one of the very few that is), the name Poseidon, along with his title Earth-Shaker, appears in Mycenean texts from very early times. On the Messenian coast, at least, he seems to have kept his status, and like Zeus he is the father of kings and heroes. (Indeed, the number of his offspring is second only to Zeus’.)
