Sources : Mule

Herodotus [c. 484 – c. 425 BCE] (Histories, Book 4.30) But I hold it strange (for it was ever the way of my history to seek after subsidiary matters) that in the whole of Elis no mules can be begotten, albeit neither is the country cold nor is there any manifest cause. The Eleans themselves say that it is by reason of a curse that mules cannot be begotten among them; but whenever the season is at hand for the mares to conceive, they drive them away into the countries of their neighbors, and then send the asses to them in the neighboring land, till the mares be pregnant; and then they drive them home again. - [Godley translation]

Pliny the Elder [1st century CE] (Natural History, Book 8, 69): [Book 8, 69] A mare coupled with an ass after twelve months bears a mule, an animal of exceptional strength for agricultural operations. To breed mules they choose mares not less than four or more than ten years old. Also breeders say that females of either genus refuse stallions of the other one unless as foals they were suckled by females of the same genus as the stallions; for this reason they stealthily remove the foals in the dark and put them to mares' or she-asses' udders respectively. But a mule is also got by a horse out of an ass, though it is unmanageable, slow and obstinate. Also all the foals from old mares are sluggish. ... Male foals of an ass by a horse were in old days called hinnies, while the term mules was used for the foals of a mare by an ass. It has been noticed that the offspring of two different races of animals belong to a third kind and resemble neither parent; and that such hybrids are not themselves fertile: this is the case with all kinds of animals, and is the reason why mules are barren. ... A mule can be checked from kicking by rather frequent drinks of wine. ... She-mules bred from a mare and tamed wild-asses [onagers] are swift in pace and have extremely hard hooves, but a lean body and an indomitable spirit. But as a sire the foal of a wild-ass and a domestic she-ass excels all others. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 1.57): The mule [mulus] has a name derived from Greek – for it is this in Greek – or because, forced by the miller’s yoke, it draws the slow millstone ]mola] around in grinding [molere]. The Jews claim that Ana, Esau’s great-grandson, was the first to have herds of mares mounted by asses in the desert, so that new animals, mules, might be conceived contrary to nature. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Quadrupeds 4.68): The mule, as he says [i.e. Liber rerum] is an animal of extraordinary strength in labor, and is born from from the adulterous mixture of the seeds of the ass and the horse. It has the characteristics of an ass: the long ears of an ass, an ugly coat, a cross on the shoulders, small feet and a lean body; but the rest it has like a horse. Mules and similar animals which are generated from different species do not have their own species; whence, if they are left, they fail of themselves. But there are truly species and not others, which if left, are by nature perpetual. A mule can never conceive. Hence it is that their nature by no means receives the property of generation from the posterity of the seed. For it is always necessary that in the procreation of a mule, an ass and a horse should come together. But there is a reason why a mule cannot conceive, since the animal was born contrary to nature, and its nature is diversified in the horse, which is hot by nature, and in the ass, which is extremely cold. It is not possible, therefore, in a diversely generated and ill-complexioned foetus, that its parts be concordant and nature ordered, so as to beget, as if from its own substance, something similar to itself, which is known to have no proper substance of its own kind, although it was begotten from the proper substances of a horse and an ass by a friendly nature. The more a mule drinks when it is a youngster, the bigger and stronger it becomes. Pliny says: As it is born of two different species to become a third species and to be similar to neither of its parents, it is certain that this does not reproduce as in every other species of animals. In the annals of the Romans it is said that mules often gave birth; but this is a prodigy. As Pliny says, a kicked mule is restrained by frequent drinking of wine. Mules are born from donkeys and mares, and colts from donkeys and horses. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.71): A Mule is called Mulus, and hath that name of Molendo, grinding, for he is under the yoke of Bakers, and draweth about milstones, as Isid[ore] saith libro. 15. And the Jewes tell, that Ana Esaus nephew, made first Asses and Mares, for to have first against kinde, the kinde of mules bred and gendred as he saith: and therefore the Mule followeth the kind of the Mare, and is more than an Asse, & fairer, and swifter: but he is more slow, fouler, and lesse than an Horse, and so the mule is a barren beast, and neverthelesse a noble beast to travaile, as Plinius saith lib. 8. ca. 44. And these beasts, the Mare and the Asse desire never to gender together, except they be together in youth, and sucke togethers while they be coltes: therefore Heards put and set their coltes to sucke Asses, and Asse colts to sucke Mares, when they will have such beasts gendered betwéene beasts of divers kinde as he saith. Also he saith: that wine drinking is forbidden ye Mule. Of wilde Asses and Mares, are swifte Mules gendred, with hard féete and able to runne, and have great rivells in the body, and are wilde in heart, and neverthelesse gentle: and those that be gendered betwéene a wild Asse and a Mare, passe all other. Libro septimo Aristotle speaketh of the Mule and saieth, that the more water that the mule drinketh, the more good his meate doth him. Also li. 14. the Mule hath no gall openlye séene upon his liver. Also lib. 21. he saith, for the mule is gendered betwéene the Asse and the Mare, he gendereth not, for the kinde of either of them, of the Asse and of the Mare is colde, and so the coldnesse of the sire and of the dam hath masterie in the mule that is gendered, and therefore the mule is barren, and nothing is gendred of his séede, by reason of passing colde that hath mastrie on him. Also, there it is sayd, that it hapneth, that bodies of Mules be great and huge, for menstruall superfluitie passeth into nourishing and féeding of the body, and the bloud that néedeth not to kinde, passeth out with urine, & therefore ye male mules smell not to the urin of ye female mules, as other beasts do that have hoves: and the other deale of superfluitie passeth into increasing and greatnesse of the body. Therefore the female mule conceiveth but by hap and full selde, and the male Mule for he is the more hot, because of the male kind, gendreth somtime in some countries and lands, and that but by hap, but what he gendereth is straunge and occasion, as he yt is gendred betwéene an horse and an Asse, and is worthye that such a one be barren, for he is gendered against kind. Huc usque Arist[otle] li. 16. Isaac in Dietis saith, that mules flesh is worse to nourishing and defieng than Asses flesh; his dirt stamped and burnt stauncheth bloud, if it be tempered with vineger, as Dioscorides saith, and the same dirte helpeth against stinging of of Scorpions, as he saith. - [Batman]