The adjective Headless makes the wielder lose their head. Strangely, a Headless Humanoid can still wear hats, glasses, and facial hair. It seems as though the stomach area is considered as the head, as headgear is placed around it.
A Shoggoth will lose the antennas on the top part of its body, this is because the head is attached to the body, and not a seperate piece.
Dragons can still breathe fire without their heads.
A Hydra, Cerberus, Scylla, or Nuckelavee will lose all three (four in Scylla's case and two in Nuckelavee's case) heads instead of just one.