A donkey is not a horse, although they belong to the same family of animals, equidae, however they are different species.
Donkeys tend to be smaller than horses and have longer ears and a distinctive bray. A donkey has 62 chromosomes while a horse has 64.
Donkeys and horses can breed, with a male donkey and female horse producing mules, and less commonly, female donkeys and male horses producing hinnies. Mules and hinnies are usually sterile and so typically produce no offspring.