The worship of Agathos Daimon was major part of the domestic cultus. Zeus also had a prominent place in the domestic cult. Pausanias is the only one I know of who actually conjectured that Agathos Daimon was an epithet for Zeus. I believe that it is because of the similarity of their roles that the two became associated.
Agathos Daimon means “the good spirit”. He protected and ensured the prosperity of the family by aiding them with good luck, good health, wisdom and abundance of food and drink. After meals, he would receive a libation of unmixed wine in a gesture of thanksgiving. The most common representation for Agathos Daimon was the house snake. Snakes were a common feature in Greek homes. The kept down the rodent population and were seen as avatars of Agathos Daimon. Many ancient household altars that have been found have paintings of a snake on them and are sometimes labeled with Agathos Daimon.
Another representation for the Agathos Daimon was a young man with a cornucopia and bowl in one hand and a poppy and ear of grain in the other hand. The companion or spouse of Agathos Daimon is Agatha Tykhe. Agathos Daimon was variously described as the head of the family, the male energy or the culmination of all the male ancestors of the family. Together with Agatha Tykhe, they would symbolize procreation and continued existence.
Zeus in domestic cult had several roles. There was Zeus Pater, Father of Gods and Men. There was Zeus Herkeios, Zeus of the Courtyard or of the Fence, who guards the house from outside dangers. There was Zeus Kataibates, Zeus who descends, who protected the house from lightening strikes. There was Zeus Ktesios, Zeus of the pantry, who protected the food stores from thieves. There was also Zeus Meilikhios, Zeus the propitious one; Zeus Soter, Zeus the Savior; Zeus Philos, Zeus the friendly; and Zeus Xenios, Zeus the Hospitable. All of these epithets involved protection and bounty. Of these Ktesios, Meilikhios and Soter were also symbolized as a snake. At symposiums, Zeus Soter would receive the first and third libations and there has also been found some home altar dedications to him.
So Agathos Daimon and the domestic roles of Zeus are very similar in form and function. Did one define the other or just overlap until they were blended together? It is hard to say for sure. At some point they did become acutely associated with one another. There is a votive relief dated the middle of the 4th century BCE of a family of worshipers approaching a three figures with a dedication on it to “Zeus Epiteleios Philos and to Philia, the mother of the god, and to Agatha Tykhe, the wife of the god”.
There is another relief during the same time period showing Agathos Daimon with Agatha Tykhe (who is holding her veil towards that Agathos Daimon in a gesture suggesting their familiarity) and Philia.
Like any divinity, Agathos Daimon is associated with other gods: Serapis, Aion, Osiris, Dionysos and Helios to name a few. But in no place did I find evidence of an affiliation that was stronger, in my opinion, than the one with Zeus. There is also those that say that each person has their own agathos daimon or guiding spirit commonly called a guardian angel. But then Pindar says that “The great mind of Zeus guides the daimon of the men he loves.” (Pythian Odes 5.122-3) So while it is entirely possible that “agathos daimon” became a catch phrase for any helpful being, THE Agathos Daimon is still Zeus. Or maybe, just maybe, my prejudice is showing. : )
See http://sannion.livejournal.com/762588.html for a lovely prayer “To Agathos Daimon”.
Amy C. Smith, “Athenian Political Art from the Fifth and Fourth Centuries BCE: Images of Political Personifications,” in C.W. Blackwell, ed., Demos: Classical Athenian Democracy (A. Mahoney and R. Scaife, edd., The Stoa: a consortium for electronic publication in the humanities [www.stoa.org]) edition of January 18 2003.
Marthin P. Nilsson, Greek Folk Religion.
Edward’s mighty brain
Numerous websites and books (thanks to Google books) that all said pretty much the same thing so I can’t point to any one in particular.