Melanism has been described as the increased development of the dark-colored pigment melanin in the skin or hair, basically the opposite of albinism. The cross fox from the image is a partially melanistic colour variant of the red fox , which has a long dark stripe running down its back, intersecting another stripe to form a cross over the shoulders(wikipedia).
The melanism can be seen also on other different animals. The black panther is a typical exemple of melanistic animal. We can ad to the list snakes, squirels, wolfs, king pinguins, zebra.