Eumetazoa

The mule deer (Odocoileus hemionus) is a deer indigenous to western North America, named for its large mule-like ears. There are believed to be several subspecies, including the black-tailed deer. Unlike its cousin, the white-tailed deer, mule deer are generally more associated with the land west of the Missouri River, and more specifically with the Rocky Mountain region of North America. Mule deer have also been introduced to Kauai (Hawaii) and Argentina. The most noticeable differences between whitetails and mule deer are the size of their ears, the color of their tails, and the configuration of their antlers. In many cases, body size is also a key difference. The mule deer's tail is black-tipped, whereas the whitetail's is not. Mule deer antlers are bifurcated; in other words, they "fork" as they grow, rather than branching from a single main beam, as is the case with whitetails. Each spring, after mating season, a buck's antlers start to regrow almost immediately after the old antlers are shed. Shedding typically takes place in mid February, with variations occurring by locale. Although capable of running, mule deer often prefer to stot, with all four feet coming down together.

A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny (the offspring of a male horse and a female donkey). While there is no known instance of a male mule siring offspring, female mules have on very rare occasion given birth to viable offspring. The size of a mule and work to which it is put depends largely on the breeding of the mule's dam. Mules can be lightweight, medium weight, or even, when produced from draught horse mares, of moderately heavy weight.

An aficionado of the mule claims that they are "more patient, sure-footed, hardy and long-lived than horses, and they are considered less obstinate, faster, and more intelligent than donkeys."

A female mule that has estrus cycles and thus, in theory, could carry a fetus, is called a "molly" or "Molly mule," though the term is sometimes used to refer to female mules in general. Pregnancy is rare, but can occasionally occur naturally as well as through embryo transfer. One of several terms for a gelded mule is a "John mule."

Deer (singular and plural) are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, mule deer such as black-tailed deer, elk, moose, red deer, reindeer (caribou), fallow deer, roe deer and chital. Male deer of all species (except the Chinese water deer) and also female reindeer grow and shed new antlers each year. In this they differ from permanently horned animals such as antelope; these are in the same order as deer and may bear a superficial resemblance. The musk deer of Asia and water chevrotain (or mouse deer) of tropical African and Asian forests are not usually regarded as true deer and form their own families, Moschidae and Tragulidae, respectively.

The word "deer" was originally broad in meaning, but became more specific over time. In Middle English der (Old English dēor) meant a wild animal of any kind. This was as opposed to cattle, which then meant any sort of domestic livestock that was easy to collect and remove from the land, from the idea of personal-property ownership (rather than real estate property) and related to modern chattel (property) and capital. Cognates of Old English dēor in other dead Germanic languages have the general sense of "animal", such as Old High German tior, Old Norse djur or dȳr, Gothic dius, Old Saxon dier, and Old Frisian diar.