When he told me this, I thought that the word basically only occurs in the phrase 'denizens of the forest', so it constitutes a fixed phrase, just like cosh only occurs in the phrase 'under the cosh', and whet rarely occurs outside of 'whet your appetite'. But I just did a search in the COCA corpus, where I found 421 instances of denizens, and only three of them are collocated with forest. So it seems that the word has a wider usage than I thought. Instances include:
Denizens of the city came runningI wonder whether I have any word that I tend to use regularly in my academic articles, as a fingerprint of my authorship. I can't think of any at the moment, but I suspect I do.
the indulgent, everhopeful denizens of Wrigley Field
the denizens of my city's different neighborhoods
the mordant denizens of Wonderland still basking in peculiarity
Land's End's denizens indeed excited alarm
He imagined the denizens of that garden cheering him