The mule deer ("Odocileus hemionus") is a species of deer. They are sometimes called black-tailed deer. They live in western North America. Their name was given to them because of their mule-like ears. There are many subspecies, including the black-tailed deer. Unlike the white-tailed deer, mule deer lives on land west of the Missouri River. They also live in the Rocky Mountains region of North America. Mule deer have been introduced to Argentina.