Rancor as in resentment or rancor as in these?
Rancors were large carnivorous reptomammals originating from the planet of Dathomir. They were usually born brown, but in special circumstances, such as the mutant rancor , Jungle rancor , and the Bull rancor, their color may have differed. Although found on other worlds such as Ottethan, Carida, Corulag, Felucia and Lehon (where they were brought by crashed starships), those from Dathomir were said to be stronger and more intelligent than others. They have been used for many things, ranging from mounts for the Witches of Dathomir to pets for crime lords such as Jabba Desilijic Tiure to being a source of food, and as a mean of entertainment... by dropping someone into its pit.
If it's the second, Obama's an even bigger nerd than I thought...
[BWAHAHAHA xpost!]