Isidore of Seville [7th century CE] (Etymologies, Book 12, 1.61): Among the animals those born of differing species are called hybrids [bigener], such as the mule from a mare and a [male] ass, the hinny [burdo] from a male horse and a female ass, the hybrida from wild boar and domestic sows, the tityrus from a ewe and a he-goat, and the musmo from a female goat and a ram. This latter is the leader of the flock. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Quadrupeds 4.51): Ibrida is a hybrid four-footed animal, that is, born of different animals, as Isidore says. Just as a mule is born from a horse and an ass, so a hybrida is born from wild boars and domestic pigs. A titurus is born from a female sheep and a male goat, but a musmo is born from a female goat and a ram. When animal matings take place, the kidneys and hindquarters of the females are covered, so that the males do not avoid them because of their disparate species. - [Google-assisted translation]