Hippomancy is the art of divination through the horse, whether it involves interpreting the animal's movements or neighing, the tracks it leaves, or its bones. Throughout much of history, the horse was seen as an intermediary between Man, Nature, and the gods. The horse was thought to have diviner or oracle powers, often as part of cults.

According to Georges Dumézil, hippomancy was widespread among Indo-Europeans in very early antiquity. Documents and testimonies refer to Romans, Persians, Celts, Germanic, and Slavic peoples. Germanic and Slavic hippomancy rituals have many points in common, in particular the sacralization of a horse that is exceptional in terms of size and coat, and that lives near a sanctuary. These rituals were opposed by Christian evangelists in the Middle Ages. Most hippomancy cults disappeared.

Today, hippomancy still plays a role in dream interpretation. The vision of omens in the attitude of a horse and the belief in its power of divination remain commonplace, particularly in the countryside of Germanic countries during the 19th century, in Central Asia, and in the Ozarks mountains in the United States today. The lucky charm attributed to the horseshoe could be linked to hippomancy.

The term "hippomancy" comes from the Greek hippos, meaning "horse", and manteia, meaning "divination", which gave rise to mancie in Old French and Middle English. The CNRTL defines hippomancy as "divination by the neighing and movements of sacred horses". More generally, The Encyclopedia of Divination describes it as "the observation of the actions of a horse followed by their interpretation as an omen of the future". For Marc-André Wagner, hippomancy in the strict sense of the word must be ritualized and stems from a vision of the horse as a messenger animal for divinities or other higher powers. In its broader sense, it also includes the interpretation of bones (osteomancy), dreams, and even objects associated with the animal, such as horseshoes. Those who practice hippomancy are known as "hippomancers". Hippomancy can involve a variety of divination techniques, including the interpretation of footprints or parts of a horse's body, such as the skull. According to Marc-André Wagner, the appearance of the horse in a dream, as in reality, gives rise to a variety of interpretations, both positive and negative.

Most ancient historians attribute importance to the prescience of horses, and hippomancy was widely practiced in the Indo-European area until the Middle Ages. The most common form involves a live horse, while scapulimancy is much rarer. Most of the time, hippomancy involves a human interpretation of the horse's movements. It also happens -much more rarely- in certain mythological tales and stories, that horses themselves speak to prophecy. The theme of the talking horse, which probably originated in animism, is not always linked to hippomancy. The horse's head is particularly important as an instrument of divination. These ritual practices were opposed by Christianity.

While the ancient Greeks seem to have been unaware of ritual hippomancy, Latin sources attest to the importance the Romans attached to equine predictions, particularly in the context of warfare. The Romans' defeat at the hands of the Parthians was predicted by the behaviour of Crassus' horse and that of Lucius Caesennius Paetus, which were said to have bolted when crossing the Euphrates. In Virgil's Aeneid, Anchises sees four white horses grazing and interprets this as an omen of war, adding that peace is still possible because horses can harness themselves to a chariot and be docile. According to the same work, Carthage was founded on the site where the exiles of Tyre unearthed a horse skull at the suggestion of Juno, a sign of war victories and abundance for centuries to come. Cicero mentions the horse in his treatise on divination, citing the Second Punic War. Gaius Flaminius is said to have fallen senselessly with his horse in front of a statue of Jupiter Stator, sparking the suspicion of his troops, who saw this as a bad omen and asked him not to engage in combat. He took no notice of this and sought the opinion of his pullary (divination by sacred chickens), who confirmed his troops' fears. He went into battle anyway but died and his army was defeated by Hannibal. In the Iliad, Achilles' horses Balius and Xanthus are gifted with prophetic speech. When Achilles returns to battle, determined to avenge Patroclus, Xanthe lowers his head and lets his mane hang down, while Hera has just endowed him with human speech. He announces that he can do nothing to change Achilles' fate, reminding him of his imminent death at the hands of "a God and a man". However, this case of a horse speaking to prophecy is very rare.

The Greco-Latin sources about Alexander the Great, particularly Plutarch and the Novel of Alexander, present Bucephalus as a monstrous anthropophagous horse, with a Pythia predicting that only Alexander would be able to ride it. The pseudo-Callisthenes version recounts that Bucephalus, accustomed to feeding on human flesh, neighed softly when he saw Alexander, recognizing him as his master. This is a combination of hippomancy and the common theme of the untamable horse that can only be mastered by a great conqueror. Horses are also capable of predicting the death of monarchs: according to Suetonius, the horses freed by Julius Caesar behind the Rubicon stopped feeding and shed tears before their master died.

Herodotus and Ctesias attest to hippomancy among the Persians, where it continued until the Sassanid era. Georges Dumézil sees it as a possible Indo-European rite of enthronement. It reflects the great importance of the horse in Persian thought (the future Iranians), and perhaps the role of diviner accorded to military riders. However, Persian hippomancy was also part of the first Indo-European tripartite function, kingship. According to the Histories of Herodotus in the 6th century BC, Darius exploited the Persians' belief in hippomancy to ensure his royal legitimacy: the six nobles decided to let fate decide who would be king, declaring that whoever's horse was the first to neigh at sunrise would be made king of the Persians. Darius (or his groom) used a ruse to get his stallion to neigh first. After his accession to power, he had a bas-relief engraved with an inscription saying that he owed his kingship to the merit of his horse and that of his squire Oibares. It is possible that the motif of hippomancy was added at a later date to the story of Darius' accession to power (or misunderstood by the Greeks), because it fits in with the vision he wanted to impose, that of the chosen one of the divinity Ahura Mazda. It is also possible that Darius really did use this ruse, or propagated the story to appease his people, who believed it.