I like the concept as much as I dislike the name. Not that I can come up with anything better, but I’d have preferred a reference to their 2D origins rather than lack of skeletal structure. Anyway I thought they carried off the pretty bonkers concept pretty well in the end.
The Boneless, so named by the Doctor, were a group of beings from a two-dimensional universe. Draining dimensional energy from the TARDIS they began to adapt to a three dimensional plane until they were sent back by the Doctor who had tried to negotiate with them until they continued to gain energy by killing humans.