Hinny
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Family: Equidae
Genus: Equus
Species: E. caballus + asinus
Binomial name
Equus hinny?

A hinny is the offspring of a male horse and a female donkey (jennet or jenny).

Hinnies compared with mules
Hinnies are rarer than mules, which are the offspring of a male donkey and a female horse. The hinny is sterile with only one recorded exception. (Ref: http://en.wikipedia.org/wiki/Mule, http://www.geocities.com/moredonkeys/aboutmules.html)

Similarities
Hinnies are similar to mules in that they are generally more intelligent than horses, and more cooperative than donkeys. Both are also healthier and less expensive to feed and maintain than horses. This is a trait these hybrids get from their donkey heritage. The donkey is a notoriously hardy creature that, in the wild, survives on a harsh diet in a desert environment.

Differences
Hinnies are on average slightly smaller than mules. There is much speculation as to the size variances among the two types of hybrids. Some fanciers believe it is merely physiological, due to the smaller size of the donkey dam versus a large horse mare. Others claim it is genetic. The view of the American Donkey and Mule Society is, "The genetic inheritance of the hinny is exactly the same as the mule."

Hinnies are smaller because donkeys are, for the most part, smaller than horses, and equine offspring will not grow larger than the mother, in this case, a donkey, can accommodate. Hinnies do however, like mules, come in many sizes. This is because donkeys come in many sizes, from miniatures as small as 24 inches (610 mm) at the withers, to Mammoth donkey jacks and jennies that may be over 15 hands, approximately 60 inches (1524 mm) at the withers. Thus, a hinny is restricted to being about the size of the largest breed of donkey. Mules, however, have horse females as mothers, so they can be as large as the size of the largest breed of horse. There are some huge mules, mostly from work horse breeds such as the Belgian.

Other than size, there are some minor differences that occur frequently between mules and hinnies. The head of a hinny resembles that of a horse, more so than mule heads do. Hinnies often have shorter ears, although they are still longer than those of horses, and more horse-like manes and tails than mules do. They often come in horse colors, as the male parent often determines the color of the coat. Therefore, mules usually have donkey coat colors. Certain traits, like the popular gait that some horses and donkeys possess, seem to pass more readily though the male parent. Therefore, many people have tried to produce gaited hybrids by using gaited male horses on female donkeys in hopes of creating gaited hinnies.

Fertility, sterility, and rarity
Hinnies are difficult to obtain because of the differences in the number of chromosomes of the horse and the donkey. A donkey has 62 chromosomes, whereas a horse has 64. Hinnies, being hybrids of those two species, have 63 chromosomes and are sterile. The uneven number of chromosomes results in an incomplete reproductive system. According to the ADMS, "The equine hybrid is easier to obtain when the lower chromosome count, the donkey, is in the male. Therefore breeding for hinnies is more hit-and-miss than breeding for mules."

Male hinnies and mules are usually castrated to help control their behavior by eliminating their interest in females. The male hinny or mule can and will mate, but the emission is not fertile. There are no recorded cases of fertile male hinnies or male mules.

Female hinnies and mules are not customarily neutered, and may or may not go through estrus. Female mules have been known to produce offspring when mated to a purebred horse or donkey, though this is extremely uncommon. Since 1527 there have been more than sixty documented cases of foals born to female mules around the world. In contrast, according to the ADMS, there is only one known case of a female hinny doing so, and she produced a mix which has, thus far, only been documented once among the offspring of female mules:

Mule mares pass along 100% of their maternal genes to their offspring, rather than a mix. Since a mule's mother is a horse, as a rule mule mares pass genes which are 100% horse to their foals. Thus, a mule mare bred to a horse stallion will produce a foal which is 100% horse, with no donkey genes at all.
In China in 1981, a hinny mare proved fertile with a donkey stallion. Scientists expected a 100% donkey foal if the female hinny had passed on her maternal chromosomes the same way as female mules do. However, when the Chinese hinny was bred to a donkey jack, she produced "Dragon Foal," who resembles a donkey with mule-like features. Dragon Foal's chromosomes and DNA tests confirmed that she was a previously undocumented combination, not 50% donkey, from her donkey father, and 50% horse, from her hinny mother. Instead of each gene pair being donkey-horse, with one left over, Dragon Foal has a combination of gene pairs that are donkey-horse and donkey-donkey, with one left over. Her hinny mother passed along a combination of horse and donkey genes.
In Morocco in 2003, a mule mare bred to a donkey stallion produced a male foal that was 75% donkey and 25% horse, rather than the expected 50%/50% if the mule mare had passed on her maternal chromosones, which are 100% horse, in the usual way. DNA testing revealed the Moroccan foal is a mixed karyotype hybrid like the Chinese hinny offspring, Dragon Foal. This means that, unlike regular hinnies, whose 63 chromosomes consist of 31 pairs that are horse-donkey with one left over, the Morocco colt has about 23 pairs of chromosomes that are donkey-donkey, eight pairs that are horse-donkey, and one left over.
Because of the mix of gene pairs in the Moroccan foal, it's unknown whether Dragon Foal's genetic oddity is due to her mother being a hinny rather than a mule, or if there is some other factor that applies equally to Dragon Foal in China and the 2003 colt in Morocco.

There are other reasons for the rarity of hinnies. Female donkeys, jennies, and male horses, stallions, are choosier about their mates than horse mares and donkey jacks. Thus, the two parties involved may not care to mate. Even if they do cooperate, female donkeys are less likely to conceive when bred to a horse than horse mares are when bred to a donkey. Breeding large hinnies is an even bigger challenge, as it requires a jenny of Mammoth donkey stock. Mammoth donkey stock is becoming increasingly rare and has been declared an endangered domestic breed. Fanciers are unlikely to devote a Mammoth jenny's valuable breeding time to producing sterile hinny hybrids when Mammoth females are in high demand to produce fertile pure-bred Mammoth foals.

http://en.wikipedia.org/wiki/Hinny

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