I'm guessing it's the Unversed version of a Darkside, but it was scrapped then the definition of what an unversed was was finalized.
Notice how it fades in dark mist(like a heartless) and gives out no heart? Heartless(emblems) spew out a big heart when they die, nobodies fade in white mist, this thing faded in dark mist and didn't let out a heart, just like an unversed.
Darkside=Heartless
Twilight Thorn=Nobody
That Thing=Unversed