From the American donkey and Mule Society:
Donkeys, zebras and mules all differ somewhat from horses in conformation. The most noticeable difference is of course the ears. Donkeys ears are MUCH longer in proportion to their size than a horse’s. The necks are characteristically straighter in the longears, and most donkeys and all zebras lack a true wither. The croup and rump are also a different shape in the donkey and it’s hybrids, lacking the double-curve muscled haunch. The back is straighter due to the lack of withers. Dipped or swayed backs are a conformation fault, unless in old animals or brood jennies who have produced many foals, and not due to genetic factors. The mane and tail in the donkey are coarse. The mane is still and upright, rarely laying over, and the tail is more like a cow's, covered with short body hair for most of the length, and ending in a tasseled switch. Donkeys do not have a true forelock, although sometimes the mane grows long enough to comb down between the ears toward the eyes. Because the mane is stiff and sometimes flyaway, many donkeys, especially show stock, wear their manes clipped short or shaved close to the neck. Hoof shape varies as well, donkey hooves are smaller and rounder, with more upright pasterns. The legs should have good bone, but many donkeys of common breeding may appear to have long thin legs with tiny feet. Larger Asses such as the Poitou or Andalusian types may appear opposite, with huge, heavy shaggy legs and large round feet. Good legs and feet are essential for breeding Mules, as a good foot is much preferable to a large body on tiny stick legs and feet. The vocal qualities are the frequently remembered differences in the long-ears. The donkey’s voice is a raspy, brassy Bray, the characteristic Aw-EE, Aw-EE sound. Jacks especially seem to enjoy braying, and will "sound off" at any opportunity. Although many donkeys are the familiar gray-dun color, there are many other coat shades. Most donkeys, regardless of coat color, will have dorsal stripes and shoulder crosses, dark ear marks, as well as the "Light Points" -- white muzzle and eye rings, and a white belly and inner leg. Leg barring ("garters" or "zebra stripes" may be present as well. Small dark spots right at the throatlatch, called "collar buttons" are a good identifying marking and occur occasionally. These typical donkey markings may be passed on in part or in whole to Mule or Hinny offspring. Colors in the donkey range from the gray shades of gray-dun to brown, a rare bay, black, light-faced roan (both red and gray), variants of sorrel, albino-white (also called cream or white-phase), Few-spot white, and a unique Spotted pattern. True horse pinto, horse aging gray, horse Appaloosa, palomino and buckskin do not occur in the donkey. The more unusual colors are the Dappled Roan, where the face and legs are light and the body is marked with "reverse" dapples (dark spots on a light background, as opposed to the horse dapple where the dapples themselves are light on dark), frosted gray (with light faces and legs and some white hairs in the coat) the pink-skinned, blue-eyed albino white, and the few-spot white. The few-spot white is off of spotted lines, and can throw either more Few-spots or true spotted colts. The animals are best defined as a spotted animal where the skin is spotted but the color does not necessarily show through on the coat. Few-spots can be identified from albino white by checking the skin around the eyes and muzzle. Albino/creams will have blue eyes and true pink skin, while few-spots will have dark eyes, dark "eyeliner" and dark spotting on the skin. Another unusual variant of the spotting line is the "tyger spot" pattern. These donkeys vary from the typical large spots over the ears, eyes, and topline. The body will be covered with small round spots resembling the Appaloosa type. Donkeys come in a variety of sizes from the Miniature Mediterranean (under 36 inches) to the elegant Mammoth Jackstock (14 hands and up). The rare French Poitou donkey, characterized by it’s huge head and ears, and very thick, shaggy, curled black coat, can stand 14 to 15 hands high. (There are fewer than 200 purebred Poitous left in the world today.) The types of donkeys are labeled by their sizes: 36" and under, Miniature Mediterranean; 36.01-48", Standard; 48.01" to 54 (jennets) or 56 (jacks), Large Standard; and 54/56" and over, Mammoth Stock. Donkeys are healthy, hardy animals but should receive the same vaccinations and wormings as a horse. Their hooves also need periodic trimming. They often live for 25 or more years. Donkeys can be used just like horses under saddle and in harness, although donkeys are more laid back and self-preserving in nature. They prefer to do what is good for the donkey, which is not always what the human thinks is best (especially when it comes to getting their feet wet.). They are very friendly, and their nature makes them excellent for children. Donkeys can perform all the gaits horses or mules do (yes, some are even "gaited", exhibiting a single-foot gait), but galloping is usually not on the program unless dinner is being served. Donkeys can also make wonderful guard animals -- a donkey gelding or jennet will take care of an entire herd of cattle, sheep or goats -- the natural aversion to predators will inspire the donkey to severely discourage any canine attacks on the herd. Dogs and donkeys usually don’t mix, although they can be trained to leave the house or farm dog alone!