I know in prehistoric times, dragonflies were enormous.
Prehistoric is when humans existed but were not recording events. Giant insects existed in the Paleozoic more than 250 million years ago, which is approximately 250 million years before humans.
I was wondering if the prehistoric mislabeling originated on the Internet and found that Google listed a couple of similarly malformed questions: "Why were insects bigger in prehistoric times?" and "How big were bugs in prehistoric times?". Check out this webpage at the usually technically savvy EarthSky website: Why were prehistoric insects so big?