Even-toed ungulates, order Artiodactyla, is a famous mammalian order with about 220 species. It includes pigs, peccaries, camels, hippos, deer, chevrotains (mouse deer), pronghorn, giraffe, antelopes, goats, sheep, and cattle. The order includes many economically useful mammals, including pigs, sheep, goats, and cattle. Another feature unique to the even-toed ungulates is the presence of a specially-shaped bone in the ankle joints, which give these animals greater leg flexibility.
The ancestry of even-toed ungulates lies in superorder Laurasiatheria, the superorder of mammals that have their origin in Laurasia, a former supercontinent that consisted of North America merged together with Eurasia. Like many other mammalian orders, they evolved in the early Eocene (around 54 million years ago). The common ancestor of living even-toed ungulates probably resembled the mouse deer of today -- which oddly look like a cross between a small deer and a rodent. These animals grew from rodent-size after the extinction of the dinosaurs to occupy the small browser and medium-sized herbivore niches. A few lineages, like pigs and peccaries, eventually became omnivores, and a few ancient relatives of modern pigs were actually effective hunters (mesonychids).
The first three branches of the even-toed ungulates to evolve were camelids (the first ruminants), suines (pigs and relatives), and suborder Ruminantia (deer, cattle, goats, sheep, antelope, etc). All these suborders were present by the late Eocene (46 million years ago). During most of the Eocene, even-toed ungulates were less numerous than odd-toed ungulates (ancestors of today's horses and rhinos). Odd-toed ungulates were well adapted to digesting lush foliage, while even-toed ungulates thrived on the frontiers, where there was harder-to-digest plant matter like grasses.
Around this time, the Azolla event occurred, which sucked most of the carbon dioxide out of the air and lead to a world temperature crash. Many of the world's great forests and rainforests died off, and grasses spread across the land. This is why the Cenozoic is called the "Age of Grasses." Perfectly positioned to take advantage with their multi-chambered stomachs and cud-chewing adaptive strategies, even-toed ungulates slowly became the most dominant herbivores on the planet, as they remain today. Horses, rhinos, and tapirs (odd-toed ungulates) have become marginalized.