The white stag...

White Stag or Hart

White deer hold a place in the mythology of many cultures. 
The Celtic people considered them to be messengers from the otherworld
The Celts believed that the white stag would appear when one was transgressing 
a taboo, such as when Pwyll tresspassed into Arawn's hunting grounds.  

Arthurian legend states that the creature has a perennial ability 
to evade capture; and that the pursuit of the animal represents mankind's 
spiritual quest. t also signalled that the time was nigh for the knights 
of the kingdom to pursue a quest