Rancors were big brown things that were giant and were covered in teeth and claws.
They evolved to become so large so that any non-force-sensitive that had the misfortune to face them become so scared out of their minds that they are rendered immobile by their fear, and are then promptly eaten.
They ate everything they could find, but they especially liked hot Twi'lek slave girls, who were often victims of having relations with rancors. (Can't blame the rancor, Twi'lek slave girls are so hot!). Rancors often developed a craving for spicy food after relations, prompting them to devour the Twi'leks because they are so hot, and are noted by those lucky rancors to have a sort of red/orange/yellow/green/blue/purple pepper taste.
Jabba the Hutt owned a rancor and would feed it anyone who pissed him off (quite a number of people).
The Witches of Dathomir were known to rape some rancors for fun. Until the rancors turned the table, raped them and then ate them. What makes this even more deliciously comedic is the fact that those rancors were bulimic, so the Witches of Dathomir were swallowed whole, puked up, then re-eaten, several times before finally being slowly digested. On his trip to Felucia, Darth Secret was attacked by many rancors. He PWN3D every one.