Elasmotherium (wiki)
“There is nearby a wide steppe, and there dwells, it is told, an animal smaller than a camel, but taller than a bull. Its head is the head of a ram, and its tail is a bull’s tail. Its body is that of a mule and its hooves are like those of a bull. In the middle of its head it has a horn, thick and round, and as the horn goes higher, it narrows (to an end), until it is like a spearhead. Some of these horns grow to three or five ells, depending on the size of the animal. It thrives on the leaves of trees, which are excellent greenery. Whenever it sees a rider, it approaches and if the rider has a fast horse, the horse tries to escape by running fast, and if the beast overtakes them, it picks the rider out of the saddle with its horn, and tosses him in the air, and meets him with the point of the horn, and continues doing so until the rider dies. But it will not harm or hurt the horse in any way or manner.
The locals seek it in the steppe and in the forest until they can kill it. It is done so: they climb the tall trees between which the animal passes. It requires several bowmen with poisoned arrows; and when the beast is in between them, they shoot and wound it unto its death. And indeed I have seen three big bowls shaped like Yemen seashells, that the king has, and he told me that they are made out of that animal’s horn.”
Ibn Fadlan, medieval traveller




