47 species of deer, elk, moose, caribou, muntjac, roe deer, wapiti, and many more cloven-hoofed ungulates are all actually deer. Whitetail deer have been around as a species for 3.5 million years, making them the oldest living deer species on Earth.

Deer are the only animals who grow antlers. Antlers grow faster than any other living tissue in the animal world. Chinese water deer are the only deer who do not grow antlers. Instead, to attract mates, the males develop and display very long canine teeth. This suggests that they are a very primitive form of deer that evolved before the evolution of antlers.

Every year 1.5 million deer-vehicle collisions occur in the United States resulting in 150 human deaths, along with, most likely, 1.5 million deer deaths.