The Flying creatures were unnamed bird or bat-like creatures that were seen somewhere in the caverns under Derceto. They do not represent a real danger but appear in areas where you need to maintain balance and the attacks of these creatures tend to drop the player. Based on their appearance - Round, bulbous head, wings, snall legs, etc. and presence in a game focused on the Cthulhu Mythos, it is more than probable these are the Mi-Go, the fungi from yuggoth.