The donkey is difficult to obtain (propagated) due to the difference in chromosomal number of horse and donkey. A donkey has 62 chromosomes, while a horse has 64. A donkey is a hybrid of two species, has 63 chromosomes and is sterile. The uneven number of chromosomes leads to an incomplete reproductive system. Crossbred horses are easier to obtain when the lower chromosome number, donkeys, are in males. The male mule or mule can and will mate.
Males and mules are often castrated to help control their behavior by eliminating their benefits in females. Deceit and yell are not castrated, and may or may not go through estrus. Mules have been known, on rare occasions, to produce offspring when mating with a mare, although this is extremely rare. There are other reasons for the rarity of donkeys. Females and stallions may be more selective about their mates than mare and male donkeys.
As such, the two parties involved may not be interested in mating. Even if they are uncooperative, a female donkey is less likely to conceive when raising a horse than a horse when mating with a donkey. Raising a large mule is a bigger challenge, as it requires genes from a large donkey, such as the Baudet de Poitou or the American Mammoth donkey. The ancient Mammoth trick is becoming increasingly rare and has been declared an endangered breed.
See more at: Donkey Hello Sweet Cheeks Donkey Poster