The boogeyman is a monster popularised by folklore and used by parents to scare their children into good behaviour: "go to bed on time or the boogeyman will get you!" Chances are everyone had some experience with this growing up. It’s the monster living in the cupboard, or under your bed.
Of course, boogeymen are an idea the film world has latched onto with great effect, since it's the perfect movie monster. What these beasts actually are or look like is vague, so filmmakers can get creative and insert whichever creature they see fit.
It's a ready-made premise, capturing the horror of discovering the monsters we heard stories about as children are real when we grow up.
And most importantly, boogeymen seem to have a preoccupation with rules: they both love to abide by rules and they’re also concerned that you stick to their rules or else you’ll run afoul of them.
John Wick might be the man you send in when you need to kill the boogeyman, but it seems none of the characters in the following films had his number...