The first bogeyman that ever existed was the primitive anthropomorphic personification of fear, the darkness in the caves, the beasts outside the caves, the terrors of the night. This is explored in great depth in Hogfather, where Susan Sto-Helit actually meets it in the Tower of Teeth. Whilst it had the power to become your worst nightmares (which is why children are so scared of it), it eventually grew fond of the little ones (as Death has grown fascinated by humans) and tried to keep them safe by the extremely primitive magical means of owning their teeth.
The original Bogeyman seems to have been very important in terms of the development of humans - indeed, was it not the fear of the dark that led to humanity fleeing into caves, nurturing fire, creating shelter and, ultimately, to civilisation?
Whilst the original has moved on to other things, other Bogeymen have appeared, and taken on the duties of scaring children, continuing the practice of peopling the dark with things you should fear.
By the time Susan met it, it was a very sickly thing - a sort of wizened monkey (formed in the shape of the proto-humans who first felt the fear of the enormity of the universe as Death and the others have such very human characteristics?) and, shorn of its protective layers of imagery, was helpless. Just like the Hiver, all it really wanted was to be shown the way.
Known bogeymen include Schleppel and Shlimazel - note the extreme Yiddishness of the names. To "schlep" is "to lug awkwardly" or "to trudge"; whilst a "shlimazel" is "a very unlucky or inept person who fails at everything".