There's still considerable debate about the first proto-monster. Was it a shapeshifter? A ghoul? Some sort of space squid? But there's strong scientific agreement that the most recent subspecies are The Monster Under The Bed and the Monster In The Closet.
These creatures have evolved to fill a very recent ecological niche- household places hidden from view but easily accessible-- and even now, have not spread to every culture. So it's understandable that their habits are less well studied than those of their more famous Elder Kin.
Still, certain truths are known. They are definite, distinct creatures. The Monster Under the Bed had developed long claws and unspeakable, unfolding arms to reach from its confines and snare its prey. Like other cave dwellers, it has no sight and relies instead on smell, hearing, and the ineffable Monster Sense that lets these hunters know when the most vulnerable member of the household is alone. The Monster in the Closet has bright reflective eyes, to take advantage of ambient light in its dark surroundings, and very sharp teeth in what may or may not be a muzzle.
Both can be evaded by the deployment of protective blanketing, and widespread Blanket Awareness has resulted in an almost total elimination of home-based monster attacks, not just in first world nations, but in every bed- or closet- having culture. Yet one point of concern remains: the Monster Under the Bed has a sharply defined territory, never reaching farther than perhaps two feet around its lair. But the Monster In The Closet has been reported behind shower curtains, in pantries, and even behind briefly open doors, its presence detected by the unerring human ability to Detect Monsters (located in the amygdala). This raises the troubling possibility that the Closet Monsters have become home-based generalists, able to occupy any domestic space not currently in view of two or more humans. If this is true, it could easily ambush prey far from reach of a blanket. It could even be behind you right now.
I do so hope we'll both be here Monday.