Rancors were large, carnivorous and semi-sentient reptomammels orginating from the planet of Dathomir. They are always born brown, but in some cases, such as the albino rancor above and the mutant rancor, the colour may have differed. Although they also originated from Felucia and Lehon and brought by crashed starships to Otthetan, Carethan and Corulag, the rancors from Dathomir were said to be stronger and more intelligent then others. It has been used for many things, ranging from the mounts of the Witches Of Dathomir to pets for crimelords such as Jabba The Hutt to being used as a food source.
Like any species of creature, the rancor is also divided into subspecies which are the bull rancor, the Coloi, the gnarled rancor, the jungle rancor, the mutant rancor mentioned above, the rancor pygmy, the tyrant rancor, the Tra'cor and the Chrysalide .
This is just a brief summary so go to Wookieepedia The Star Wars Encyclopdia for more details because that is where I got this information.