As for the nature of the monster, it’s somewhat reminiscent of the titular entity in The Babadook and others, in that it feeds on the family’s trauma and suffering; it can be interpreted as a manifestation of the family’s hurt and grief.

Source: https://centralrecorder.com/what-is-the-post-credits-scenes-of-the-boogeyman-movie-what-to-watch-and-who-is-the-monster/

Right now it is “I am your Mother!” from The Babadook.

Source: https://www.thecourier.co.uk/fp/entertainment/4334215/dundead-dalila-droege/