Star Wars: Jedi vs. Sith: The Essential Guide to the Force by Ryder Windham, pages 198-200
Rancors pop up in this book in the section titled Other Force-Using Organizations as part of a summary of Force Wtches. These witches lived in enclaves on various planets and drew their power directly from the light side of the Force. On the other side were the Nightsisters, a group of witches that had embraced the dark side and were exiled to form their own clan. The most noteworthy of the Force Witches were the ones on the planet Dathomir. See Rancor Spotting #9 for more information about Dathomir.
The book points out that a banished Jedi Knight named Allya tamed the rancors on Dathomir to do her bidding and instituted a symbiotic relationship between rancors and Force Witches that continues to this day. It also informs us that a rancor named Tosh, herd leader of the Singing Mountain Clan’s rancors, provided the following narrative, which had been passed down over many generations.
Listen, children, to the story of how our ancestors first met the Witches.
A sickly female rancor met a Witch who healed her, and the Witch rode on the rancor’s back, and learned to speak the rancor’s tongue. By riding the rancor’s back, the Witch was able to spot food better with her sharp eyes that see well even in the daylight, and the rancor thrived and became huge. In time, she became a herd mother, and her herds prospered while others died out.
Back then, the rancors did not know how to make fine weapons like spears or nets. They did not know how to protect themselves with armor. Because the Witches have taught them such great things, the rancors must always love the Witches and serve them, even when they make unreasonable demands to give them rides through the wilderness or ask us to fight the Nightsisters.
Never serve the Nightsisters, children. They’ll treat you badly, as if you were mere slaves. Their taste is foul, so it is best to crush them. If you cannot crush them, then escape from them, for then you can continue to serve the good Witches, and hope to crush the Nightsisters another day.
Try to imagine a rancor covered in armor and brandishing a spear. Whew!