The boogeyman was a monstrous character in Earth culture. The bogeyman did not have a given appearance or characteristics, but was the embodiment of fear.
In 2370, as Jean-Luc Picard finished a conversation with Mr Boothby, the Starfleet Academy groundskeeper, the gardener warned Picard not to let the boogeyman get him. Moments later Picard was attacked by a Tellarite assassin. (TNG comic: "An Inconvenient Truth")