Horses and donkeys are both equines, but they are quite different from each other, both physically and mentally.
Physical Differences Between a Horse and a Donkey
Donkeys have larger ears than horses, they have a thin mane, and no forelock, their tail is sparse. They have a longer back in proportion to their legs, and their heads are larger in proportion to their body than that of a horse. Donkeys do not have chestnuts on their hind legs and the chestnut on their front legs tends to be larger than that on a horse. Their hooves are more upright than a horse and tend to be more oval rather than round at the bottom. A gray donkey is actually gray and does not change color like a gray horse would.
When a donkey gets fat its neck accumulates fat in the crest, horses get fat all over. Donkeys are more thrifty on feed and more prone to obesity and founder if overfed.
Donkeys tend to have a flatter croup than horses, and lack the fifth lumbar vertebra.
Mental Differences Between Horses and Donkeys
Donkeys are somewhat different to train than horses are, they respond well to rewards and do not respond well to punish. A donkey might even refuse to move forward if whipped, whereas most horses would leap forward to get away from the pain. A donkey will usually not go someplace it thinks is unsafe, they like to stop and think about what they are going to do before they do it.
Other Differences Between Horses and Donkeys
Horses neigh, donkeys bray. A bray is loud and long.
Donkeys have a body temperature that changes more in the day, being lower at night, whereas a horse's body temperature remains fairly constant.
Donkeys have 62 chromosomes, horses have 64. They can breed together, producing an often sterile mule, who has 63.
Donkeys are pregnant for roughly a year, while horses are pregnant for only 11 months.
Donkeys have longer lifespans than horses.
Donkeys have good endurance but are not as physically as strong as a horse (as may be related to their longer back).
Donkeys tend to walk most places, where as horses are quite happy to trot and canter.
Author and her husband have owned several horses and donkeys, and currently own a standard donkey.