Equidae
Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus "Equus". Equidae belongs to the order Perissodactyla, which includes the extant tapirs and rhinoceros, and several extinct families.
Elk
Until recently, red deer and elk were considered to be one species, "Cervus elaphus". However, mitochondrial DNA studies, conducted on hundreds of samples in 2004 from red deer and elk subspecies as well as other species of the "Cervus" deer family, strongly indicate that elk, or wapiti, should be a distinct species, namely "Cervus canadensis". The previous classification had over a dozen subspecies under the "C. elaphus" species designation; DNA evidence concludes that elk are more closely related to Thorold's deer and even sika deer than they are to the red deer. Elk and red deer produce fertile offspring in captivity, and the two species have freely inter-bred in New Zealand's Fiordland National Park, where the cross-bred animals have all but removed the pure elk blood from the area.