Okay, just wait a while...
...runs of for a hipster costume together with false beard, thick glasses ...
Hmmm... let's see. I think that bogeymen are a personification of existential uncertainty in the emergent Dwarf Fortress narrative apparatus. Their makeshift "to-be-finished-later" cheesiness is actually a masterful touch, contrasting to otherwise coherent and consistent laws of DF gameplay mechanics. They are intentionally crude, to make you ask question, to ponder about the never-ending Ouroboros-shaped cycle of intention and consequence, the meaning, and the function, the expression, and the impression, or if you like, the yin and the yang. By facing his inner-most fears in the depths of the night, the player experiences not catharsis, nor enlightenment, nor any form of insight, but a brutal and naturalistic death instead. Yet there is much symbolism in a dwarven or human skull, caving in after a random punch. The crushed brain is an especially powerful symbol, referring to sensory overload, the failure of human mind to nervously catalogize the onslought of external stimuli the life itself. On the outside, the skull shatters and the brain pours through the newly created holes like a thick, grey goo. On the inside, the human ego disintegrates, unable to maintain the constant self-deception we inflict upon ourselves - the ever present illusion of the individual ego.
Only the id remains, like a gaping hole to an alternate dimension, with an endless stream of pure horror pouring out of it.
Thus, in a nutshell, a bogeyman is a representation of the ultimate futility of human (or dwarven) struggle for self-definition. It's what remains, when you remove everything else - preferably with a very sharp blade.