Sir James George Frazer (1854–1941).  The Golden Bough.  1922.

§ 2. Killing the Sacred Ram
THE RUDE Californian rite which we have just considered has a close parallel in the religion of ancient Egypt. The Thebans and all other Egyptians who worshipped the Theban god Ammon held rams to be sacred, and would not sacrifice them. But once a year at the festival of Ammon they killed a ram, skinned it, and clothed the image of the god in the skin. Then they mourned over the ram and buried it in a sacred tomb. The custom was explained by a story that Zeus had once exhibited himself to Hercules clad in the fleece and wearing the head of a ram. Of course the ram in this case was simply the beast-god of Thebes, as the wolf was the beast-god of Lycopolis, and the goat was the beast-god of Mendes. In other words, the ram was Ammon himself. On the monuments, it is true, Ammon appears in semi-human form with the body of a man and the head of a ram. But this only shows that he was in the usual chrysalis state through which beast-gods regularly pass before they emerge as full-blown anthropomorphic gods. The ram, therefore, was killed, not as a sacrifice to Ammon, but as the god himself, whose identity with the beast is plainly shown by the custom of clothing his image in the skin of the slain ram. The reason for thus killing the ram-god annually may have been that which I have assigned for the general custom of killing a god and for the special Californian custom of killing the divine buzzard. As applied to Egypt, this explanation is supported by the analogy of the bull-god Apis, who was not suffered to outlive a certain term of years. The intention of thus putting a limit to the life of the human god was, as I have argued, to secure him from the weakness and frailty of age. The same reasoning would explain the custom—probably an older one—of putting the beast-god to death annually, as was done with the ram of Thebes.   1
  One point in the Theban ritual—the application of the skin to the image of the god—deserves particular attention. If the god was at first the living ram, his representation by an image must have originated later. But how did it originate? One answer to this question is perhaps furnished by the practice of preserving the skin of the animal which is slain as divine. The Californians, as we have seen, preserved the skin of the buzzard; and the skin of the goat, which is killed on the harvest-field as a representative of the corn-spirit, is kept for various superstitious purposes. The skin in fact was kept as a token or memorial of the god, or rather as containing in it a part of the divine life, and it had only to be stuffed or stretched upon a frame to become a regular image of him. At first an image of this kind would be renewed annually, the new image being provided by the skin of the slain animal. But from annual images to permanent images the transition is easy. We have seen that the older custom of cutting a new May-tree every year was superseded by the practice of maintaining a permanent May-pole, which was, however, annually decked with fresh leaves and flowers, and even surmounted each year by a fresh young tree. Similarly when the stuffed skin, as a representative of the god, was replaced by a permanent image of him in wood, stone, or metal, the permanent image was annually clad in the fresh skin of the slain animal. When this stage had been reached, the custom of killing the ram came naturally to be interpreted as a sacrifice offered to the image, and was explained by a story like that of Ammon and Hercules.   2