Sumatran Short-Tailed Python (Black Blood): Python curtus
Ash is an example of a naturally occurring melanistic animal. They are also know as the Black Bloods. Melanistic animals have higher levels of dark pigment (melanin) in their skin. There are some examples of melanistic individuals found among wolves, cats, and even birds. The melanistic Sumatran short tails are an example of a race of animals in which the trait is more than just a rare occurrence, but continued on through generations of wild breeding animals.
There are two races of wild Sumatran short tails. The northern Sumatrans have an orange head, whereas the southern Sumatrans will have either black or silver head.
Northern 
Southern
Photo credits: Brian Cimala
Compare the coloration of Ash in the first picture, at the top of this page, to the one right here. These two photos show the power of line breeding. Ash (top photo) is an older animal from earlier generations. This younger animal here has achieved much darker coloration through several generations of controlled breeding of exceptionally dark individuals back to other dark ones.
