Cthonians were among the legendary or “fictional” creatures kept in the Zoo of the Strange and Wonderful House. An old, experienced zookeeper once told a young recruit that one of the pieces of knowledge from works of fiction which she'd find came in handy as a zookeeper in this particular Zoo was “where to tickle a Cthonian”. (PROSE: Our Strange and Wonderful House: The Zoo)
Behind the scenes[]
This appears to be a misspelling of Chthonians, huge, burrowing eldritch worms from the Lovecraftian Mythos.