Saw an unusual deer recently...wonder if anyone else has seen one?

I've read that this occurs about once in every 20,000 animals. I'd say you were fortunate to encounter one. Usually you'll see it repeated in a local herd as the gene gets passed from generation to generation. Here in RI I'm aware of at least two separate herds with the piebald gene showing up every now and again. Here's a little info / video on the topic: