A hinny is a hybrid species of equine with the opposite genetic pairing of a mule. Mules are jack donkeys bred to horse mares; hinnies are stallions crossed with jennet (or jenny) donkeys. Both mule and hinny are gender-neutral terms. Hinnies are less common than their mule counterparts, but more common that most people suspect because not everyone realizes they are not the same thing as mules. Both must be bred from existent horse and donkey populations because all males and 99.9% of female mules and hinnies are sterile. Females will occasionally give birth; males have never on record sired a foal, so the lines must be maintained from non-hybrid animals.

Hinnies are less-often bred than mules for several reasons. First and most importantly, they're harder to breed. The higher-chromosome horse conceives easily from the lower-numbered donkey, but jennys have a much more difficult time conceiving by the higher-numbered stallion sire. Hinnies are also often smaller than mules because the mother is a smaller animal herself. Mules also often display a phenomenon called "hybrid vigor which causes them to grow larger than both parents--hinnies do not exhibit this phenomenon. Some people think hinnies are weaker or less intelligent than mules, but there is no clear evidence for this. This theory probably comes from the hinny's smaller size and finer bones giving the impression of a weaker animal. They are, however, generally slightly less nervous and stubborn than mules, having more of a donkey temperament than higher-strung horse characteristics.

Hinnies and mules are often near-impossible to tell apart by sight, but some characteristics may help. As stated, they are usually smaller. They also tend to resemble horses more than mules do. They often have finer heads, shorter coats, smaller and sometimes rounder ears, and longer, thicker manes and tails than mules. But as with every species and especially with hybrids, there is a large spectrum of appearances for both each parent species and both hybrids. Behaviorally, mules more often bray and hinnies more often whinny. A "field test" can be performed, releasing an animal into a mixed group of donkeys and horses. Mules usually congregate with horses and hinnies with donkeys, as they have 'implanted' on the species of the dam. While slightly more reliable than some visual clues, it is by no means a guarantee.

Because it is so hard to distinguish a mule from a hinny without knowing its true parentage, mules and hinnies are classified together in work, show, and registry books. These animals are classified by size (ranging from pony-like to draft animals) rather than species because there are so many types of horses and donkeys but all yield the same offspring, genetically, and breed specifics generally do not show in the hybrids at all.