Sometime in the 1960s, Post cereal got the idea to make a cereal based on Casper the Friendly Ghost, it was to be called “Ghosties”. Casper had two cartoon series airing on TV at this time (one which reran the old theatrical shorts, and one with all new content), so putting out a breakfast cereal at this time made sense. They made a (barely) animated storyboard for a commercial for this cereal, but never a full commercial. It seems that they never made any of the actual cereal either. According to the commercial, it would have been corn based and dipped in honey (probably close to Sugar/Honey Smacks or Golden Crisp).
This predates the conceptually similar Boo Berry ceral from General Mills, which didn’t come out until 1973.
Why this cereal never made it to market is a mystery.