Yes, it is possible for some speceis to develop traits that are unusual compared to the other members of their species. These changes, or Mutations, as they are properly called, are exceedingly rare. an example would be a 2 headed turtle, or a one eyed mouse. Although mutations occur naturally, through genetic malfunction, they can also be produced scientifically, most often to breed animals that can be used for marketing. such as cows being bred for higher quality milk, or some such. In any case, it is in fact quite possible, just rare, for a Raven to have white feathers.
