I don;t think there's a well-defined answer. The "capacitance of a cat" is actually the capacitance of the cat and some other condictor. For purposes of calculating the cat's capacitance you can treat the other plate as a grounded sphere at infinity. It doesn't really matter., you'd get pretty much the same answer with a grounded sphere a few feet away. But for figuring out how much energy you can store before you get an arc, it's all about how far away the second conductor is.