"Ungulates" are, broadly speaking, animals with hooves. They can be further divided into "odd-toed ungulates" (such as horses, tapirs, and rhinoceroses) and "even-toed ungulates" (such as pigs, giraffes, deer, cattle, antelopes, goats, sheep, hippopotami, and camels), and even into smaller categories, including whales, dugongs, aardvarks, and elephants. This is one of the most diverse and successful groups of mammals on earth today.
For the purposes of D&D, the "ungulate" category generally includes creatures that are quadrupedal, herd-based herbivores. Generally inoffensive, especially large, aggressive, or well-trained ungulates can become a problem for adventurers, and those that aren't a direct threat may belong to some NPC's (as domesticated creatures), or otherwise shape the campaign world. They often make excelent mounts, and can be domesticated with little effort.
Ungulates share several traits in common:
Ungulates generally prefer to flee rather than attack, and are often quite quick on their feet, and very aware of the world around them, with sharp hearing and scent (though their vision tends to be weak). When provoked, they may lash out with hooves or large feet, try a savage bite, or ram or gore an enemy with elaborate horns. They can also be trained to fight by adept beastmasters.
Ungulates are generally easily spooked, and flee from or become aggressive at the slightest disturbance. Given their large size, this can become a problem. Others are gentle and easygoing, however, especially around a skilled handler. They are almost all prey animals, and so are always on the look out for creatures that might try to devour them.
Ungulates follow elder members of the herd, often previous generations, sometimes ancient matriarchs. They may also follow a suitably powerful beastmaster, who bends them to respond to her will above all.
Most ungulates are herbivores, and the quality of traveling in herds means that they strip an area clean of most plantlife fairly quickly, and then move on to better grazing grounds. They are migratory creatures by an large, and long distances don't bother their great endurance