From
Robert Heinlein's novel
Tunnel In The Sky. When Rod Walker (our protagonist) is sent to a far off
planet for
survival training, he is warned by his
teacher to "Beware Of Stobor".
When he first arrives, he hears strange voices. He assumes the makers of these frightening noises are the dangerous stobor. A little later he discovers that the noises are actually made by small, unthreatening, lizards. Later he concludes that the large, leonine, creatures he encounters are the real stobor.
This view continues for some time. During this time, Rod discovers an animal which is christened a "Dopey Joe", a small, generally good-natured, creature. Then the season changes and the Dopey Joes go crazy, violently attacking anything in sight. Rod concludes that these are the mysterious stobor. He walls his settlement and surrounds it with traps to catch the stobor (nee Dopey Joes).
When he is finally returned home, he mentions the stobor to his teacher. The teacher claims he had no idea what the stobor were like. Stobor are, he explains, the idea, the personification, of the unique dangers of each planet in the galaxy. Students are warned to "Watch out for stobor" to keep them on the edge, looking for the hidden dangers of their situation.
Rod: "Well, I'll be a- Then there weren't any stobor! There never were!"
Teacher: "Sure there were. You built those traps for them, didn't you?"
Watch out for stobor.