Ammon was the Graecized name of a major Egyptian deity, in the native language Amun, "the hidden one" (alternative spelling "Amon"). He was one of the Ogdoad. He was married to Ipet.
Amon and Mut
It was not, however, until the rulers of the XVIIIth dynasty carried their victorious arms beyond the Egyptian frontiers in every direction that Ammon began to assume the proportions of a universal god for the Egyptians, eclipsing all their other deities and asserting his power over the gods of all foreign lands. To Ammon the Pharaohs attributed all their successful enterprises, and on his temples they lavished their wealth and captured spoil.
Ammon is figured in human form, wearing on his head a plain deep circlet from which rise two straight parallel plumes, perhaps representing the tail feathers of a hawk. Two main types are seen: in the one he is seated on a throne, in the other he is standing, ithyphallic, holding a scourge, precisely like Min, the god of Coptos and Chemmis (Akhmim). The latter may be his original form, as a god of fertility, before whom the king ceremoniously breaks up the ground for sowing or cuts the ripe grain. His consort was sometimes called Amaunet (feminine of Amun), but more usually Mut; she was human-headed, wearing the double crown of Upper and Lower Egypt, and their son was Chons.
The name of Re, the sun-god, was sometimes joined to Ammon, especially in his title as "king of the gods": the rule of heaven belonged to the sun-god in the Egyptian cosmos, and this identification with Re was only logical for a supreme deity. Ammon was entitled "lord of the thrones of the two lands," or, more proudly still, "king of the gods." Such indeed was his unquestioned position when suddenly he was overthrown and his worship proscribed. Not even a henotheist fervently worshipping one of many gods, Amenhotep IV of the XVIIIth dynasty became the monotheist Akhenaten; discarding all the gods of Egypt, and especially persecuting Ammon; he devoted himself to the purer and more sublime worship of Aten, the sun. But he failed to win the permanent adhesion of the people to his reform, or to conciliate or entirely crush the enormously powerful priesthood of Ammon.
A few years after the reformer's death, the old cults were re-established and the monuments of Aten studiously defaced. Hymns were then addressed to Ammon-re, which are almost monotheistic in expression. The cult of the supreme god spread throughout Egypt and was carried by the Egyptian conquerors into other lands, such as Syria, Ethiopia and Libya, and was accepted by the natives both in Ethiopia and in the Libyan cases, where civilization was low and Egyptian influence permanent. After the XXth dynasty the centre of power was removed from Thebes, and the authority of Ammon began to wane. In the XXIst dynasty the secondary line of priest kings of Thebes upheld his dignity to the best of their power, and the XXIInd dynasty favoured Thebes: but as the sovereignty weakened the division between Upper and Lower Egypt asserted itself, and thereafter Thebes would have rapidly decayed had it not been for the piety of the kings of Ethiopia towards Ammon, whose worship had long prevailed in their country. Thebes was at first their Egyptian capital, and they honoured Ammon greatly, although their wealth and culture were not sufficient to effect much. Ammon (as Zeus) continued to be the great god of Thebes in its decay, and notwithstanding that a nome-capital in the north of the Delta and many lesser temples, from El Hibeh in Middle Egypt to Canopus on the sea, acknowledged Ammon as their supreme divinity; he probably in some degree represented the national aspirations of Upper Egypt as opposed to Middle and Lower Egypt: he also remained the national god of Ethiopia, where his name was pronounced Amane. The priests of Amane at Meroe and Napata, in fact, regulated through his oracle the whole government of the country, choosing the king, directing his military expeditions (and even compelling him to commit suicide, according to Diodorus) until in the 3rd century BC Arkamane (Ergamenes) broke through the bondage and slew the priests. Ammon had yet another outburst of glory. There was an oracle of Ammon established for some centuries in Libya, in the distant oasis of Siwa. Such was its reputation among the Greeks that Alexander the Great journeyed there, after the battle of Issus and during his occupation of Egypt, in order to be acknowledged the son of the god. The Egyptian Pharaohs of the XVIIIth dynasty had likewise been proclaimed mystically sons of this god, who, it was asserted, had impregnated the queen-mother; and on occasion wore the ram's horns of Ammon, even as Alexander is represented with them on coins.
The Egyptian goose (chenalopex) is figured in the XVIIIth dynasty as sacred to Ammon; but his most frequent and celebrated incarnation was the woolly sheep with curved ("Ammon") horns (as opposed to the oldest native breed with long horizontal twisted horns and hairy coat, sacred to Chnumis). It is found as representing Ammon from the time of Amenhotep III onwards.
As king of the gods, Ammon was identified by the Greeks with Zeus and his consort Mut with Hera. Chnumis was likewise identified with Zeus probably through his similarity to Ammon; his proper animal having early become extinct, Ammon horns in course of time were attributed to this god also.
Alternative: Kematef (much later in Egyptian history)
See Erman, Handbook of Egyptian Religion (London, 1907); Ed. Meyer, art. "Ammon" in Roscher's ''Lexikon der griechischen und römischen Mythologie;'' Pietschmann, arts. "Ammon," "Ammoneion" in Pauly-Wissowa, Realencyclopädie.
Initial text from 1911 encyclopedia -- Please update as needed
Several words derive from Ammon: ammonia and ammonite. Ammonia, as well as being the chemical, is a genus name in the foraminifera. Both these foraminiferans (shelled protozoa) and ammonites (extinct shelled cephalopods) have/had spiral shells resembling a ram's, and Ammon's, horns. Ammonia the chemical derives its name in a more round-about way - see end of article ammonia.