Horses are the species Equus caballus, while donkeys are classified as Equus asinus. Since both species are of the Equus genus, they have the same basic body shape and share many physical traits, including long muzzles and pointed ears.
Horses are known for their smooth coats, flowing manes and graceful stature, while donkeys often have wiry or bristly hair and a spiky mane. On average, donkeys grow to about 550 pounds, but the extensive breeding of horses produces sizes ranging from 660 to 4,400 pounds, according to Animal Diversity Web. Donkeys and horses have a different number of chromosomes, making them genetically incompatible. They can mate to produce mules or hinnies, but their offspring are incapable of reproducing.