From DariaWiki

HST (or Hostile Sub-Terrestrial) is the official designation by countries which are signatories to the Bowman Acts for any being classically defined in the cinematic horror genre (such as werewolves, vampires, ghosts, mummies, incubi/succubi, and so forth).

Pseudo-vampiric beings (such as the members of Project Nightbreed do not fall under this designation, but are classified as SPBs.

The term was first used by the (now inactive) U.S. Government program known as The Initiative.