The Deer is a Shinto spirit and a messenger.
History[]
The deer appeared in many episodes of the third season of the 2012 TV series, and he has always been close to the Turtles since "Within the Woods". But only in "Vision Quest" the Deer had revealed his true form to Leonardo. He has also followed and aided Leo during his part of the Quest.
When Raphael, Donatello and Michelangelo saw his anthropomorphic form they thought he was another mutant attacking them.