Add a flag for monsters indicating “has seen/encountered the player yet”, which is set to true when first coming in line of sight of the player or being next to them. This is used to possibly change their initial peaceful/hostile state when they see you; for instance, if you’re an orc polymorphed into an elf, any orcs that generated peaceful will turn hostile when they sight you for the first time, and vice versa for elves that generated hostile.