What is a group of hoofed mammals distinguished for the thickness of their skins, including the elephant, hippopotamus, rhinoceros, tapir, horse, and hog. It is now considered an artificial group called?

pachydermata — A group of hoofed mammals distinguished for the thickness of their skins, including the elephant, hippopotamus, rhinoceros, tapir, horse, and hog. It is now considered an artificial group. (Webster's 1913.)

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