Pliny the Elder [1st century CE] (Natural History, Book 8, 30): Among the same people [Ethiopians] is also found the animal called the yale, the size of a hippopotamus, with an elephant's tail, of a black or tawny color, with the jaws of a boar and movable horns more than a cubit in length which in a fight are erected alternately, and presented to the attack or sloped backward in turn as policy directs. - [Rackham translation]

Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 52.35): There is also the eale. Otherwise like the horse, it has a tail like an elephant, is black in color, and has jaws like a boar. In front it carries horns over a cubit long. These are serviceable for use in what movement it wishes, for they are not rigid, but can be moved, as need requires, for fighting. When it fights, it extends one horn; the other it folds back, so, if the sharpened point of one is spoiled by some blow, the prong of the other succeeds it. It has been compared to the hippopotamus, and indeed, it rejoices in the water of streams. - [Arwen Apps translation, 2011]

Aberdeen Bestiary [circa 1200 CE] (folio 16v): There is an animal called the yale. It is black, as big as a horse, with the tail of an elephant, the jaws of a boar and unusually long horns, adjustable to any movement the animal might make. For they are not fixed but move as the needs of fighting require; the yale advances one of them as it fights, folding the other back, so that if the tip of the first is damaged by a blow, it is replaced by the point of the second.

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Quadrupeds 4.36): The yale, as Solinus says, is a beast like a horse, with an elephant's tail, black in color, with boar-like cheeks, having horns longer than a cubit, with the ability of obeying any movement. For they not fixed, as Jacobus says, but they are moved as usage requires for fighting; when it fights, it stretches out one, and reserves the other, so that if by chance one is cut or injured, the other can be used. This animal enjoys the waters of rivers. - [Badke translation/paraphrase]