Horse
The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), Tapiridae (the tapir), and Rhinocerotidae (the rhinoceroses)—have survived to the present day.
Aquilinae
The Aquilinae consists of medium-sized to very large species. Modern species range in mass from the recently-recognized pygmy eagle ("Hieraaetus weiskei") of Papua New Guinea, which weighs about to the martial eagle ("Polemaetus bellicosus") and golden eagle ("Aquila chrysaetos"), which both weigh about on average. In wingspan, extant Aquilinae range from the pygmy eagle, with a median of , to the wedge-tailed eagle ("Aquila audax") and martial eagles, which average about and in wingspan, respectively. Total length can vary from , in the pygmy and wedge-tailed eagles, respectively. The record sizes for wild booted eagles are for a golden eagle in body mass and for a wedge-tailed eagle in wingspan. Dwarfing these species, the booted eagle species Haast's eagle ("Harpagornis moorei") of New Zealand, was probably the most massive species ever known in the entire accipitrid family, with females averaging an estimated and perhaps weighing up to , but went extinct by the 16th century when human colonizers killed off their prey. Beyond their typically large size, Aquilinae species have few outward shared characteristics as they are a fairly diverse subfamily. Nonetheless, every species shares the feature of their legs being covered in feathers. Only two buteonine hawks share this feature beyond the Aquilinae, in a presumed case of convergent evolution. Many species within the subfamily are colored for camouflage with varying patterns of brown, black, yellowish or white being commonplace, darker colors being favored by forest dwelling varieties and brown to straw coloring common to open country species. A few more brightly colored species (in adult plumage), with striking patterns of chestnut in about four, none more striking than the ornate hawk-eagle ("Spizaetus ornatus"). Roughly half of booted eagle species have a strikingly different looking juvenile plumage, which can in some species be nearly all white, but even where the differences are subtle between adults and immatures, usually some physical differences are noticeable such as white wing patches in traditional "Aquila" species, to visually distinguish young from adults. Many species have a notable head-crest, such as "Nisaetus" and most "Spizaetus", with the feature being most extreme in the long-crested eagle ("Lophaetus occipitalis"). Like most accipitrids, they have large powerful hooked beaks for tearing flesh from their prey, strong legs and powerful talons. Compared to other large raptors, the feet and talons of booted eagle species are often particularly large and powerful relative to their size. The most extreme example seems to be the crowned eagle, which has a hallux-claw (or rear talon) of around about the same sized hallux-claw as the Philippine eagle ("Pithecophaga jefferyi") (not part of the Aquilinae), which weighs on average nearly twice as much. The booted eagles also have extremely keen eyesight to enable them to spot potential prey from a distance.