I might as well get the most out of this guest membership and ask the really important questions:
So SW fans, where is this badboy from? What’s the other life like on the planet that this creature normally inhabits?
Also, are there any creatures in the SW universe that are bigger (besides the lazy tremors monster)?
In one of the books, I believe it was The Courtship of Princess Leia, you learn that the rancors are from the planet Dathomir. And they, if memory serves (it’s been a while since I’ve read it), are at least semi-domesticated on that planet.
A lazy puppeteer at Industrial Light and Magic?
StarWars.com lists Jabba’s rancor’s origin as “unknown, possibly Dathomir,” and offers this summary of something from an EU source:
It also says that the Dathomir rancors are generally taller.
“A cross between a gorilla and a potato.” Heh.
The novelization of ROTJ claimed that it was a one of a kind mutant. Does this have to be discounted?
As far as rancors being domesticated, I believe a group of Force-sensitive witches called the Nightsisters had trained (or Force-enslaved) rancors for use as mounts.
Yep, and when domesticated, they’re actually very gentle, intelligent creatures.
I could dig out my copy of Courtship to look for a direct quote, but I’m pretty sure that Jabba’s rancor was fairly young and was raised by a handler in a pit in Jabba’s palace to be a spectacle. “Feral” would be a good description.
How that one in the sewers in KOTOR got there, I have no idea.
Someone bought it for a pet, and flushed it when it got too big?
In the book Tales From Jabba’s Palace, they also mention that some Jawas found it in the wreckage of a crashed spaceship (locked in a cargo hold, IIRC). They then sold it to Jabba.
I can’t tell you how happy it makes me to discover that someone else answered this question first – because it means there are geeks just as big as me out there.
Thank you. Thank you all. 
Silly me.
My first thought was “Well, my rancor comes from the hack job that Lucas did on Phantom Menace, and his use of offensive racial stereotypes.”
