The Indian wild ass or Indian onager is a unicorn described by Ctesias. It is as big as a horse or larger, has white coat, crimson head and blue eyes. Its horn is white at the base, red at the point and black in the middle. The horn can be made into a drinking vessel and whoever drinks from it, is invulnerable to common diseases and even poison. Indian asses have solid hooves, not cloven. Common asses lack gall bladders and talus bones, in the meaning the Ancients understood them, but the Indian ass possesses both.
The Indian ass is strong and swift; when it runs, it starts slowly, but rapidly gains in speed to the point that no horse is able to overtake it. The only way to hunt it is to catch the herd with young and surround them with many men on horses. The adults refuse to abandon their foals; they bravely defend them, biting the attackers, kicking them with their hooves and goring with horns. Nothing can withstand the power of their horns. Huntsmen fear to come to close quarters to them, for it means certain death, and instead shoot them with arrows and throw javelins at them. A fully grown ass is impossible to capture alive.
The meat of wild asses to too bitter to eat; they're hunted only for their horns and talus bones.
Herodotus writes about horned asses living in Ethiopia, but his use of the plural number in relation to the horns suggests that they aren't unicorns. The first proper description is given by Ctesias, who places their dwelling in India (there was some confusion about those two geographical areas, which persisted to late Middle Ages). Although he was considered unreliable, for many centuries we has one of the very few source of knowledge about this exotic land. Aelian adds some details, also elongating the horn from one cubit to a cubit and a half.
Aristotle generally regarded Ctesias as unreliable, but chose to include the Indian ass in his list of creatures, giving it more credibility. Pliny the Elder copied Aristotle and, as far as world science before XVIII century was concerned, if Pliny says something exists, it exists. Albertus Magnus said, probably basing it on accounts of real onagers, that these asses sometimes crush rocks with their hooves for no other reason but boasting their strength.
The authors noted the resemblance of the Indian ass to the monoceros in habitat, size, speed and alexipharmic properties of the horn. Some (like Edward Topsell) consider this the same creature, but most list them separately.
Wild ass (onager)
Ctesias calls the creature a wild ass, which is a real animal. Persian wild asses, or onagers used to inhabit areas from Palestine to the Gobi Desert. They were famed for the strength of their hind legs, utilized in a kick so mighty, a siege engine was named after them. Known from their unconquerable character, great swiftness and ferocity, they were a favorite hunting game of Persian kings. Their coloration is described as "reddish above" and "silvery grey" on the belly and hinder parts; similar basic colors to the unicorned ass and only different hue. Just like Ctesias described, normal asses have no gallbladders and their talus bones are so small Ancient Greeks did not consider them worthy to be called that name. But the Indian ass is special. It has a horn on the forehead, a gallbladder and taluses in their legs; no living equid fits that description.
The rhinoceros is the only known animal with a single horn and many books take it for granted that Ctesias was speaking about them. Indian rhinoceroses, unlike elephants, were never domesticated. A rhinoceros has a very weak sight but a keen sense of smell; when it senses danger, to throws itself to flight, not caring what it tramples; the ancients interpreted it as a frenzied attack and therefore this inoffensive animal became know for its ferocity. The skin of a rhinoceros is too thick to be pierced by arrows. Before the age of firearms, rhinoceroses were hunted in great drives led by kings, in which hundreds of men took part.
Rhinoceros is very heavy and its legs are short in proportion to its body. Thus, like in Ctesias's description, he starts slowly and gains in speed.
The Chinese believed in magical properties of the rhinoceros horn and often made beakers out of them. Red, white and black are sacred colors to the Chinese; it's reasonable to suppose that Ctesias might've seen a Chinese cup painted in those three colors.