White animals are generally a way to get another world, beyond the reality. The white deer refers to a common celtic custom: the white deer hunt. This custom is explicated in the Chrétien de Troyes' novel Erec et Enide. The first knight who kills a white deer can kiss the woman he wants among the court's ladies.

Most of the time, the white animal takes the knight far away from his starting point and disappears when the young man discovers the other world. (Note that it's a white rabbit that takes Alice in Wonderland.) The knight must pass through several proofs before he can come back to his world, generally with a reward. In Erec et Enide, Erec meets the woman he loves in this parallel world.