The expression "cavorting beasties" is not commonly used, and there is no known historical or literary source for it. It's likely a playful or humorous term that has been invented or popularized in recent times.

If you heard it somewhere, it's possible it was used as a creative or unique way to describe playful or mischievous creatures, perhaps in a story or a fictional world.

If you can provide more context about where you encountered the expression, I might be able to give you a more specific answer!