"Black fog" (tentative) is the name of a creature which was planned to appear in Resident Evil 4 during its 2002 Castle stage of development, but was eventually cut. The creature was intended to be created using a new virus developed by H.C.F. and responsible for Leon S. Kennedy's infection when both US Agents and H.C.F. had infiltrated Oswell E. Spencer's castle.
- When asked about this version, Kamiya refers to it by the name "Mist Version" (also translated as "Fog version"), referencing this creature.
- The enemy was dropped when it was realised that the level of animation needed to make this creature appear realistic was not possible on the GameCube.
- By coincidence, Black Fog is similar to the Uroboros creatures introduced in Resident Evil 5. Like Black Fog, it was also faced by the technology problem; this time Capcom had to invent a completely new animation technique to get around it.