Sentence:  Incidentally, the best official text description I know of is in Sorcerer, when you actually become a <WSD>grue</WSD> and visit a <WSD>grue</WSD> colony. IIRC, even that description is vague, but does cannonizeSIC that they are large four-legged reptiles. 
Meaning of the word marked with <WSD> tags:  A fictional man-eating predator that dwells in the dark.