There's a white, flying, mountain-dwelling, lawfull good "horse" in MM2. I don't remember what it's called, but it's very early in the book, so it's name probably starts with an A... It's the opposite of a nightmare in just anout every way. It even has the cold subtype, IIRC.
I remember it struck me at once when I first saw it, that it was the perfect mount for a paladin.