Reynard's natural form is a big, red fox - we can see it clearly when he and Antimony are in ether:
After possessing the wolf-toy, he can change shape however as he wishes, saying "body is just a plaything of a mind"
Yet almost always he assumes the form of a wolf - initially I thought its mostly because he is in a toy-wolfs body, but since the Gunnerkrig Court is so full of hidden symbolism, doesn't it have a deeper meaning? Is it because he is trying to protect Annie?