The Deer Mother was a venerated spiritual figure since early Neolithic times. She was worshipped by early peoples from the British Isles, Scandinavia, Russia, Siberia, and Mongolia. She still is revered in many places today.
On the Winter Solstice, it’s said the Deer Mother takes flight, carrying the sun’s life-giving light into the new year on her antlers.
In old finno-ugric cultures (regions from Scandinavia to Russia) deers had the power to travel between the world of the living and the world of the dead - the underworld. Shamans would turn themselves into deer and travel between the worlds in animal shape. And deers helped souls of the dead to cross the border. Winter solstice matches this idea, since it is the time when the sun is reborn and the "dead" time of year ends and a new year begins. And the deer carries the sun back from the underworld.