The Boneless was the name the Doctor gave to a race of two-dimensional beings from another plane. They were able to reduce both people and three-dimensional objects to two-dimensions. They were also able to change objects from two-dimensions into three-dimensions after taking a three-dimensional form when the 2Dis reversed its original effects.
The Boneless experimented and dissected three-dimensional people and objects, and eventually learned how to exist as three-dimensions themselves. (TV: Flatline)