The head of a deer or goat completes the Unicorn and since this is the only season in which deer or goat could be hunted, it becomes another symbol of death.  The goat is also the astrological symbol for Capricorn during which the Yew falls.  The Yew is followed by the Silver Fir of birth just as the Unicorn's Deer or Goat head is connected to its Horse body, which symbolizes birth, as the death-day in the calendar is followed by the day of birth - the Extra Day of the Year.

The Deer Mother is the primeval Birthgiving Goddess.  Her image is preserved in Scottish and Irish fairy tales of women who would transform themselves into deer, and in deer who turned into women.  This may also be a memory of the shamanka who donned deerskins as she journeyed in the Mystery Rites.  The Sumerian Goddess of childbirth was a horned deer.
The Ancient British Goddess by Kathy Jones

There is a long Celtic tradition of faerie women who appeared as deer. In Scotland, the Isle of Jura got its name from the Old Norse word for deer, dýr-ey, and was inhabited by seven huge deer-goddesses who lived there with their herds. In the Highlands, the deer-goddess was sometimes seen as an old woman tending her herds of deer in Glen Nevis, and wandering hunters sometimes heard her singing as she milked the does. 

She was an aspect of the Cailleach, the Old Veiled One, an earth-shaper who had created Scotland’s mountains and isles. As well as appearing in the shape of a deer, she was also seen as a cormorant, eagle or heron. In her human form, she had a cow which gave quantities of white milk, and her band of women rode over the country on wild pigs and wolves conjuring up storms in winter-time. Clearly she was a Mistress of the Wild Things, although in later centuries she was vilified as an evil witch.