Buchis was born from the fusion of four older bull cults that had worshipped Montu, the falcon-headed war god whose temples at Medamud, Tod, Armant, and Thebes formed the Palladium of Thebes, the sacred defensive perimeter of the southern capital. Montu was the god of the Theban nome, the district of Upper Egypt that produced the pharaohs of the Eleventh Dynasty who had reunified Egypt after the chaos of the First Intermediate Period, and he remained the divine patron of Egyptian armies for two thousand years. In the early periods, each city had its own bull that embodied Montu's power, but by the Thirtieth Dynasty, around 343 BCE, these four cults had merged into a single institution: the Buchis bull, the one living animal who stood for Montu at all four sanctuaries simultaneously.

The selection of a Buchis bull was a matter of precise theology. The priests searched the herds for a bull calf with a white body and a black face, the colours of the solar barque that carried the sun god Re across the heavens each day. The Roman author Macrobius, writing in the fourth century CE, claimed that the Buchis bull's colour changed with every hour, like the sun shifting from dawn gold to noon white to sunset red, and that its hair grew in the opposite direction to that of ordinary cattle, just as the sun moves across the sky in the opposite direction to the fixed stars. The Egyptians themselves interpreted the name Buchis in several ways: as bakhu, "the Eastern One," the rising sun; as ba-akh, "the shining soul"; and as ba-hr-khat, "the soul in the body." Whatever the etymology, the message was clear: the bull was not merely sacred. He was the sun itself, walking on four legs.

The defining feature of the Buchis cult was its underground necropolis, the Bucheum, excavated at Hermonthis by the Egypt Exploration Society between 1927 and 1930 under the direction of Robert Mond and Oliver Myers. The Bucheum was a vast catacomb of vaulted chambers cut into the desert rock, each chamber holding the mummified body of a single Buchis bull laid in a massive stone sarcophagus. When a bull died, the priests embalmed him with the same care given to a pharaoh, wrapping his body in linen and packing it with natron, the salt-like mineral that drew all moisture from the flesh. Then he was placed in his chamber and the entrance was sealed, while above ground a round-topped stela was erected recording his birth date, his installation date, his death date, and the length of his life, calculated to the exact day. The earliest known burial dates to the fourteenth regnal year of Nectanebo II, the last native pharaoh of Egypt, around 343 BCE. The last known burial is recorded on a stela dated to the fifty-seventh year of Diocletian, which corresponds to 340 CE, though by then Diocletian had been dead for twenty-eight years and the bull had actually died under the Roman emperor Constans. The cult had endured for nearly seven centuries.

Each stela tells a story. The most famous is the stela of Ptolemy V, discovered in the Bucheum and now in the Egyptian Museum in Cairo, which records that the bull buried in the twenty-fifth year of the king's reign was born in year eleven and lived for thirteen years, ten months, and twenty-eight days. The stela is painted and gilded, showing Ptolemy V in the blue war crown offering the hieroglyph for fields to a gilded statue of the Buchis bull, who stands crowned with the sun disk and double plumes while the falcon god Montu hovers behind him with wings outstretched. The winged sun disk of Horus of Edfu crowns the top of the stela, and a scarab atop a djed pillar - the backbone of Osiris, the god of resurrection - flanks the scene with protective uraei, the rearing cobras that spat fire at the enemies of the sun. Thousands of such stelae once stood above the Bucheum, a forest of stone that marked the graves of gods.

In life, the Buchis bull lived in a sacred stable at Hermonthis, fed on the finest grain, and attended by a priesthood that treated his every movement as an omen. He delivered oracles, his behaviour interpreted by the priests as messages from Montu, and he participated in bull fights that re-enacted the cosmic struggle between order and chaos. When the Valley Festival brought pilgrims from across Upper Egypt to Thebes, the Buchis bull was paraded through the streets in a golden boat, his horns gilded and his body draped in linen, the living image of the war god who had once led the armies of the pharaohs to victory. In later times, the cult expanded to include the mothers of the Buchis bulls, who were also mummified and buried in their own chambers when they died, a recognition that the divine bloodline passed through the cow as well as the bull.

The Buchis bull was also identified with the Apis bull of Memphis, the sacred animal of Ptah, the craftsman creator god whose cult was the oldest and most venerated in Egypt. This identification meant that the Buchis was eventually considered an incarnation of Osiris, the green-skinned god of the dead who was murdered by his brother Set and resurrected by his wife Isis, for the Apis bull had long been linked to Osiris in the theology of Memphis. The four bull-headed statues of Montu discovered at Medamud, each representing the god as simultaneously present at Thebes, Armant, Medamud, and Tod, embodied the four male members of the primordial Ogdoad, the eight frog- and snake-headed gods of Hermopolis who had created the world from the primeval waters. The inscriptions on these statues declare that "they are united in effigy in their form of a bull," a theological statement that made the Buchis not merely the servant of Montu but a participant in the very creation of the cosmos.

When Christianity swept through Egypt in the fourth and fifth centuries CE, the temples were closed and the old gods were abandoned, yet the Buchis cult lingered longer than most. The last stela, dated to 340 CE, proves that priests were still embalming bulls and carving stone in the name of Montu while the Roman Empire was already Christian. But the Bucheum fell silent soon after, its chambers sealed by desert sand and forgotten by the Coptic monks who built their monasteries nearby. The name Buchis survived only in the Greek and Roman authors who had marvelled at the changing colours of the bull, and in the scattered stelae that archaeologists would eventually pull from the rubble.