“Hiposolea” is a technical term used in archeology or ancient history to refer to the “sandals”, soles or protections that the ancient Greeks and Romans placed to the equine, horses or mules, in their hooves or paws
The term “hiposolea” is formed from the Greek word ἱππὁς, meaning "horse" and the Latin word " solea" meaning "sandal, sole”.
"Solea" in turn is derived from "solum", “ soil” , and generally lower something. The term “sole”is derived also from "solum”, part of the shoe that is in contact with the "floor." So says Varro in De re rustica, 1, 47
"…then the ground (foot) of man is the ruin of the grass and the foundation of a way"
"Exitium herbae Solum et enim hominis semitae fundamentum".
Several texts and some archaeological and artistic evidence revealing the existence of protections for the hooves or for the feet of the horses. They were made of metal, usually iron, and also of esparto, leather, wicker or reed. They were subject with ribbons or cords to the legs, so that placement or removal should be easy, as is clear from the text of Suetonius, Life of Vespasian, 23.2 that tells us how a muleteer stops on midway trip to supply footwear to the mules of the imperial yoke; the horseshoe operation would require more time and sufficient technical means:
Suspecting once, during a journey, that his mule-driver had alighted to shoe his mules, only in order to have an opportunity for allowing a person they met, who was engaged in a law-suit, to speak to him, he asked him, " how much he got for shoeing his mules?" and insisted on having a share of the profit. (Alexander Thomson, Ed.)
Mulionem in itinere quodam suspicatus ad calciandas mulas desiluisse, ut adeunti litigatori spatium moramque praeberet, interrogavit quanti calciasset, et pactus est lucri partem.
The feet of horses and mules should be protected from abrasive stone floors or if a disease of the hoof. Horse sandals may also favore pulling force of quadrupeds. It is used for pack animals rather than military cavalry. In any case we must consider the importance that caring for the horses hooves must have for ancient peoples and transcendence that was the adoption of the shoe for it.
I quote two texts that speak of these "hiposoleas" and the extravagance of Emperor Nero and his wife Poppaea. Says Pliny, Naturalis Historia, XXXIII, 49, 140:
…and it was in our own age that Poppæa, the wife of the Emperor Nero, ordered her favourite mules to be shod even with gold!
… nostraque aetate Poppaea coniunx Neronis principis soleas delicatioribus iumentis suis ex auro quoque induere iussit.
And Suetonius, Nero, 30, 3:
It is said, that he never travelled with less than a thousand baggage-carts; the mules being all shod with silver, and the drivers dressed in scarlet jackets of the finest Canusian cloth, with a numerous train of footmen, and troops of Mazacans, with bracelets on their arms, and mounted upon horses in splendid trappings. (Alexander Thomson, Ed)
Numquam minus mille carrucis fecisse iter traditur, soleis mularum argentéis, canusinatis mulionibus, armillata phalerataque Mazacum turba atque cursorum.
But horses hiposoleae or sandals are not exactly horseshoes, which are also a metal protection adapted to the hooves, but fastened with nails. For some time it was thought that the sandals were the antecedent of the horseshoes, but which neither the classical sources and archaeological evidence give news before the ninth century BC.
Now, it turns out that there are numerous findings of "horseshoes" in various Celtic and some Roman contexts. Arises, then, the unresolved issue of whether there were horseshoes in Roman times; question still not fully resolved.
Although some excavations, especially old, offer little scientific guarantees, it seems undeniable the appearance of horseshoes at Celtic sites in central Europe and the British Isles and the Celtiberia Hispania, especially in the provinces of Soria and Guadalajara.
The Romans soon came in contact with these people. Did you meet horseshoes? Why they did not accept these to protect their animals?
In any case it is not possible to accept the natural transition from "solea" sandal, to the horseshoe, but the coexistence in different cultural contexts. It would be right at the end of the Roman Empire when it is imposing horseshoe use, which requires a good technique and good workers, until finally prevail in the Middle Ages. No doubt new excavations and careful studies give us valuable information about it.