Lower Being was a term used to refer to certain demons who were not native to Earth and who were also particularly powerful, though not in the same level as the Old Ones. Unlike the Old Ones, the Lower Beings were also humanoid in appearance. D'Hoffryn, master of the Vengeance demons, was one of them.
Higher beings of good such as The Oracles also used this term in a pejorative manner to refer to mortal humans, even to warriors serving The Powers That Be. However, this does not necessarily entail the same meaning when the term was used to describe demons such as D'Hoffryn.